I'm Sp12 -- I write guides here and can be found streaming at http://www.hitbox.tv/sp12; if you have any questions/comments feel free to post at either. My current MMR is ~5.5K.
This is a set of builds for Sniper, one of the most straightforward carries and the hero with longest range in Dota. There's a couple of very viable ways to build Sniper: each build gets its own minisection for you to check out, so consider this an anti alt-tab guide. The formatting for this guide is matched heavily with my Faceless Void guide, which you can check out if you play him.
Sniper is fun because he has the potential to be an incredibly hard carry but is relatively powerful throughout the game, and has a good laning phase between Shrapnel, Headshot, and Take Aim, plus his ultimate, Assassinate, which gives him kill potential in most lanes at level 6.
He is relatively squishy (16 starting strength with 1.7 gain, no defensive skill), but can play with positioning to compensate. One of his big issues is playing against initiators and heroes with gap-closers, so think carefully before picking him.
I'm not going to list out skills/whatnot. There's a well-maintained wiki, great replays, and even video guides on how to play him. It's almost impossible to teach positioning via text -- an easy way to get better is to watch replays on player perspective -- filter recent games for Sniper and very high skill level.
No skill or item build is appropriate for every game. Consider these suggestions. I will try to discuss when and why you skill which way so that it makes some sense.
Shrapnel is really strong. It's over 400 AoE damage, vision, a moderate slow, and easy tower damage on 1200 range. By spamming it when it's off cooldown you do ridiculous free damage to towers. It's also your most reliable means of getting out of a gank, kiting, or pushing. Headshot is really strong at one point (40% chance for full .25 duration stun), but only becomes relevant as a damage increase once you attack quickly with items/levels.
A lot of people suggest skipping Shrapnel for 4 stat levels. While stats are very strong, I'd argue you want at least one shrapnel for the utility of the slow and vision, and it does scale with points.
In the same vein, the items are just a suggestion. If you have 3 invis heroes don't get a Shadow Blade. If they have a Sand King don't get MoM as he'll just blink Burrowstrike Epicenter you. If you're playing as a ganker Urn of Shadows makes sense. Some people demand a Magic Wand on every hero (it is pretty good). Do what works for you.
This build focuses on building movespeed items to maximize your kiting ability as well as make you difficult to gank without a lot of resources. Blink Dagger is often picked up here to help with kiting. Basically you pick this build when you would pick Sniper anyway -- they don't have any way to quickly jump on you on explode your face. It is possible to run this build mid.
Basic regen gives you a decent chance of laning well, while phase into aquila gives you some stats and damage for the early game. You get Shrapnel on this build since it's so strong for kiting, MoM is your movespeed, regen, and attack speed item, though as always it's effectiveness is directly related to the difference in skill between you and your opponents, so feel free to skip it. Try to push with team. Movespeed is strongest when you're running away into the protection of allies.
Extend into Manta Style for more movespeed and damage, then into whatever luxuries you want. Check the item rundown towards the bottom of the guide for an explanation.
This build focuses on leveraging Headshot's ministun and physical via maximizing your attack speed, and is the most stable hardcarry Sniper build. You should only run this build safelane (either solo or with a support) or trilane (either safe or hard lane). The medallion is optional but matches your range very closely and offers a huge DPS increase, as well as some mana for shrapnel.
Basic regen gives you a decent chance of laning well. If you can, you should get Hand of Midas, possibly before boots. Don't try to get it after 9 minutes, just get treads. Maelstrom gives great stats (25IAS/24damage, chain lightning) and synergizes with MoM, which is your movespeed, regen, and attack speed item. Shadow Blade is pretty strong initiation and mediocre escape on great stats, so consider it if you're the only invis hero on your team or the enemy team isn't competent enough to buy dust.
Extend into whatever you want, with BKB as my first suggestion as it gives you crucial magic immunity so they can't just explode you. Around 35 minutes there won't be time to just AFK farm so you'll probably have to play with team. Remember your positioning! Check the item rundown towards the bottom of the guide for an explanation.
This builds you a bit differently, focusing on sniper's strong intelligence growth, nuke, and shrapnel. Level 16 is kind of important on this build, and it can be run mid so that's an option to get your exp early. Midas is another.
Force is the mid-game HP regen and escape item of choice. Start building your Orchid Malevolence, which gives stupid damage output for the price as well as a very strong active. Shrapnel slow into Orchid into rightclicks into assassinate into Orchid pop is easy kills for lone targets. Make sure you spam (no really, just spam) shrapnel with this build as you'll have excess mana.
Pick from your most applicable extensions.
Sniper is kind of interesting mid. He has completely terrible base damage, no way to get to runes quickly, and is easily ganked. On the other hand, he has a sick animation, long night vision, and easy pushing with shrapnel along with long range and strong level dependance. Consider who you're laning against before going. Pudge? Your long range and night vision means you should basically get freefarm assuming you're equal in skill and don't get hooked. Queen of Pain? She has a blink and tons of burst damage, along with a great animation and base damage. Don't do it.
Your typical bottle rush. Even on mids that don't particularly care about runes, it's important to contest runes at the very least to deny them to the enemy mid. Try to have your bottle delivered by ~1:50 so you can get the rune. Make sure you spam shrapnel on the 1:45 wave meetup so that the enemy mid has to choose between experience and safe last hits or a rune. Anytime you don't need shrapnel for lane control spam it on their tower to wear it down. Basilius early gives you some needed extra damage as well as the ability to push the wave extra hard by turning your aura on.
Finish your choice of boots. Phase gives some extra mobility and damage for last hitting, treads makes your bottling more efficient by switching to agility before using bottle, int before casting shrapnel, and strength for extra durability. Drums are optional but always a solid pickup. You can gank with runes and Assassinate assuming the lane you're ganking has appropriate burst damage to go along with it. You can push pretty well with shrapnel and your long range, so do so. Make sure you pressure your own mid tower if the enemy mid isn't there. You build shadow blade for the extra ganking ability this build offers, as most teams expect contribution from their mid, something shadow blade lets you be a bit more aggressive in.
Extend into whatever luxuries you want. Check the item rundown towards the bottom of the guide for an explanation.
That's a lot of builds. You have a couple of big factors in choosing a build.
What lane would I be most effective in?
Am I the hardest carry on the map?
Do we have invis heroes?
Am I better than my opponents
If your team is up against a harder carry like Faceless Void you need to pick an early aggression build and force advantages for your team so you can end early. Euls or an attack speed build make lots of sense to help disable him in chronosphere. On the other hand, if you're the only thing resembling a carry on the map you can afford to go greedy with farming.
Be aware of your item picks. If your team has 4 invis heroes don't build shadow blade, get blink or force staff for mobility and escape. If they have squishy escape heroes go for the orchid build. If they're pushers maybe an early maelstrom for counter pushing is important. If they have a QoP don't build Mask of Madness.
I can't give you a list of every scenario and every pick, so here's an easy pro/con table for the builds to help you choose.
Moderately difficult to gank
Easy item progression
Partially online fast
Not a ton of DPS
Minimal durability, especially with MoM
Strongest carry option
Strong farming ability
Scales well lategame with open extensions
Midas gives you a fast level 16
Shadow blade split push and escape/initiation, plus it makes their supports poor
Open to ganks
Less useful if you already have an invis hero
Lose your lane early and you're probably not relevant for the next 15 minutes
Overextensions possible because you think shadow blade=invulnerability
Very strong early game
Strong farming ability with shrapnel spam
Scales well into a carry role with open extension
Mid or sidelane
Able to escape some ganks with Orchid->TP out
Minimal HP early
Potential for ganks/solo kills in lane at level 6
Very easy to gank
You don't deny most mids from getting exp
Most teams see sniper mid and want to gank you continously
Treads or phase -- tough call
Overextensions possible because you think shadow blade=invulnerability
Let me help
Blame your team.
Alternatively, look at yourself and think what went wrong and how you can get better. Consider whether your laning situation was appropriate. Not having strong lanes or lanes with a realistic farm priority, or both, (Sniper and PL in one lane?) can end your game before it really starts. Did you take teamfights against enemy teamfight ultimates instead of split pushing? Did your team gank effectively or have wards at critical times? If every lane but yours lost then even great play by just you can't win the game. If you or your team made a major mistake (5-man Reverse Polarity?) that can cost you the game. It's actually really good to think about why you lost a game afterwards as it forces you to adapt. You can check out replays in your game history.
Those are the most viable builds for our little dwarven friend. If you have any questions/comments/criticisms feel free to post them. At the very least I'd like a comment why if you feel like downvoting :P. If you feel it was worth the read a vote up helps get builds seen more.