This guide is still in progress. The basic idea is described below. The approximate times are given because I feel this is usually missing but might help alot of people to include and to show exactly what it is that is intended.
Each step is intended to prepare you for the next step, with the final goal being to dominate with very high Damage and Attack Speed, but being impossible to catch. If you are completely dominating the other team, you might be able to replace Shadow Blade.
Aim for last hits, Buy wraith band to help at the start with this. Rush Shadow Blade.
2) Mid-Early 12:00-20:00
Now with Shadow Blade, you should be able to organise a couple of ganks over 10 minutes, but first priority is probably to keep last hitting creeps, whatever helps you rush Maelstrom. Maelstrom is solely for the ability to gain incredible pushing and hence money earning power. You can kill a creep wave in a few seconds with this and a couple of damage items. Once you get Maelstrom, you should be able to earn money very quickly, that's why this is the second big item.
If you can get a couple of kills at this stage, you could be the best off in terms of gold.
Rush Daedalus and maybe Mjolinor. One option is also to increase dramatically your money earning ability by buying Boots of Travel, this allows you to push a couple of waves of creeps and maybe a tower, then as soon as you suspect the enemy coming for you, Teleport to another part of the map and push again. This is a VERY fast way of earning money, and will help your team by putting the enemy always on defense, and set you up with some awesome end-game items. An advanced tactic would be to play the bait by pushing towers and have your teammates plan an ambush.
Late Game items people will always question. But there are 5 Core items that all pay a key part in this strategy. If you choose different items, you will be choosing a different strategy.
This is really the point to which you have been building the whole game. The idea is push a creep wave back quickly, and maybe a tower, and as soon as the enemy comes close TP or SB away to kill more creeps and another tower. If you do well, you could even pick one off from a team and still SB out alive. These can all be mixed with team play, like organising distractions, being the bait, wasting their gold on gems, causing lots of Team Fight damage etc.
The reason for each Late-Game item in this strategy is as follows:
can save you many times early game, and give you the opportunity to gank early and give an escape for enemy ganks. You want to die as little as possible early game, as you need both the cash and the downtime to earn more cash.
Boots of Travel:
Kind of optional, but works really well with this strategy, should be gotten after Maelstrom and high damage items like Daedalus, as there is no point in TPing to another area unless you can kill creeps VERY quickly.
Maelstrom is necessary for this strategy, Mjolinor is not. Maelstrom is necessary for it's pushing and gold earning capabilities. but with only 6 items possible, Maelstrom should probably be upgraded, but this is not bad, Mjolinor greatly increases your attack speed from Maelstrom. Basicaly you already have Maelstrom so might as well upgrade it.
Critical hits do lots of damage, and because it is a damage multiplier, the more damage you have, the greater the critical hit. Thus, it works GREAT with Divine Rapier. Mathematicaly speaking, the average amount Daedalus increases your damage is equal to (the chance of critical hit=.25)*(the percentage increase of the crital hit=2.4)*(your base damage). This is the same as (.6) *(your base damage).
This means that if you have Divine Rapier and the other items in this guide on a level 20 Sniper, you will have about 500 base damage. What is the effect of a critical hit on this?
Thus, if your Sniper has 500 damage, Daedalus increases the damage by the same amount as a second Divine Rapier, it is also cheaper, and you don't loose it if you die!
Basicaly, with Sniper and Divine Rapier, there may be no better 2nd option than Daedalus(AEGIS does not count, because you should be playing so as not to die. This is why you have SB and maybe BoT)
Even without Divine Rapier. Daedalus still does alot of damage, and you should probably get it. It does +81 Damage and also adds on average .6 of your damage to your damage.
With this setup, Assasinate is not nearly so useful, as every shot does more damage than assasinate, and also fires much faster than assasinate.
You should get it if you are not dying often, which you should not be, you bought Shadow Blade early so that you would not die, and have as much cash as possible late game and so that things like Divine Rapier + Daedalus are no longer only dreams.
Other End Game Items
People differ on this, so I will not say much. For this playstyle you have what is necessary with five items.
Choose the 6th to max damage, maybe a second Divine Rapier? With Daedalus this will increase damage by an average of 300 + .6*300=480. You will have on average about 1000 attack. Your actual Damage will range between about 800 and 1300 on this setup.
Desolator is popular, but there are better options, like Manta. It will do more, it's not too hard to prove this, but this is counting on a late game 'I have too much money scenario'. Desolator may be helpful in many other guides, in many other games.
Satanic would be good, if you buy this you might be able to give away your shadow blade to a struggling teamate(or sell it if you are selfish).
Abyssal Blade will increase your stun ability, and hence will make it harder for enemies to run away or to catch you.
AEGIS would be good if you expect to be dying, which you shouldn't do with your SB and possibly BoT. Better off maxing DPS so that you kill them faster and you won't need to fear death. Satanic will also increase survivability. Maybe better to give AEGIS to your tank.
Manta Style. Ranged images deal .28 damage of the main hero. with 500 Attack, your two images will do a combined .28*2*500=280 damage. This is not a bad choice, the increase is about the same as Daedalus.
Blink Dagger might be fun. 3 escape items? I have never tried this, but if you are doing well it could be fun, you can bait, then you can run, you can reposition. They will hate you, and they will never kill you.
What else should you do with all the extra gold? Anybody want a Satanic?
Comparison of Items for max Damage
The following calculations neglect the fact that these items do many other great things, such as Manta intelligence and speed, butterfly attack speed, and Abyssal Blade stun. The damage also is not the only factor in calculating DPS. Damage per second is based on Damage AND Seconds, which means attack speed makes these calculations inaccurate. Also I'm not sure if Manta images use Daedalus crits?
Divine Rapier: (Base+300)
Manta Style: (Base+36)*(1+2*.28)
Desolator: (Base+60)*(1+.06) [the .06 is the armor reduction effect, note that this effect works for your allies damage too]
Abyssal Blade: (Base +110)
From this it can be shown that if your damage is greater than about 570, Manta Style and Daedalus do about as much extra damage for your than a second Divine Rapier.