I'm going to skip the notion you have no idea who Naix/Lifestealer is and how to play him and get right to the meat of this guide, which is why my recommendations are good and you should follow them.
Probably the biggest thing you'll see is that I'm not recommending Power Treads, and that is because while they are good, I feel Phase Boots with a jungle build have certain advantages. While in the jungle, the loss of 30 attack speed is probably a somewhat big deal, but if you pick up a Midas (which you should be able to) you get that attack speed back. You pick up +24 damage, which is very good early on, and you get more movement speed. This is especially useful one you move out of the jungle, as the active on the Phase Boots will give you some good chase. This is more important since the recent change to Open Wounds which now has a leveled range, meaning you have to get rather close early on to make use of it. Plus most of your items will be giving you attack speed, as will your Rage; so getting boots that make it easier for you to catch enemies and do more damage, not a bad idea.
When you go to jungle, please follow the following advice. Check the adjacent rune (top rune if you're on Dire side, Bottom if you're on Radiant) at 0:00. A rune pickup prevents the enemy mid or anyone else from getting it, and you can usually have the mid check the other, meaning, more importantly your team gets it, not the enemy team. This can be rather important in certain line-ups. Not to mention an Illusion or Double Damage rune will make killing the first camp easier. Speaking of which, try and kill the easy/small camp by ~54 seconds and move away so it can respawn at one minute. Once you do that, just go back and forth between easy/small and medium camps until you're level 5 or higher. Don't be afraid to use your ultimate to kill jungle camps, just try and use it ones with a larger number of creeps, as the instant kill and AoE effect is very useful (especially if you run a bit low on health and need a snack).
Be aware of were the enemy players are, as you will inevitably be ganked in the jungle. This is especially true in games where your team does not ward. If you are smart and/or lucky you can setup a counter-gank, somewhat early on. If the enemy mid happens to be roughly the same level as you (must be 6 or higher for this to work), and you notice him/her wander into your jungle aka farm-zone; you can do the following. Jump inside a creep, see if they come past, and if they do; jump out for the AoE damage, slow them and fire off rage. You get the idea. If you have some coordination with your team, it's even better. Just don't be mindless while jungling. Put down any Noam Chomsky or Plato you might be reading.
I've already mentioned why going Phase Boots over Power Treads is my preferred option. But let's talk about the rest. I'll assume I don't need to explain why you get the starting items and how they work.
Get Hand of Midas by ten minutes, or don't get it at all. If you can't get a Hand of Midas, I would suggest you do get Power Treads, otherwise you'll miss out on the attack speed. Buy the Gloves of Haste first and use the courier to bring them to you. This will make you farm faster, which is important. If you get lucky with creeps, you'll really not need all 3 Tangos, but if you are unlucky and get Ghost Camps, or are dumb and try farming the hard/large camps too early, you'll need them. Once you have enough gold for the recipe, buy that and use the courier. Use it immediately and whenever it comes off cool-down with the utmost of haste. Use Midas on big creeps for a bigger EXP bonus, and on small or lane creeps for more effective gold. I tend to use it on big ones until I have Boots/Armlet/Drums, then I begin laning and just use it on whatever for the most part. Gold/EXP are always good.
Get your boots next, then Armlet. The nice thing about Midas/Phase on Naix early on, is you don't need to stress picking up the components of the Armlet to increase farm. However, if you are going to, get the Gloves for more Attack Speed. Remember, all but the recipes for Midas and Armlet can be purchased at the side-shop for either set of boots and the Armlet. This is nice to remember if you join in on a gank.
If someone else on your team is going to get drums, don't get them. If nobody else is, please do. The extra stats are really good, and all items post Midas/Boots/Armlet are rather pricey so it might be wise to pick this up in the meantime. Also, if you get Power Treads and not Phase Boots, don't pick it up either, it's a waste at that point.
Once you pick up your core items. Make sure you keep a TP Scroll handy, that's important. And pickup whatever basically suits the situation, but remember there's a lot of item synergy on Lifestealer.
If you get a Desolator, you should get an Assault Cuirass next. The combined armor drop effects stack, and really turn up the damage against high-armor heroes. I see this as more of a "we're doing rather well and going to end early" option, or an option to deal with heroes who have high-armor and/or are buying lots of armor items.
If armor is not your concern, you might be better served picking up Mjolnir and a Skull Basher to turn into an Abyssal. The combo of attack speed and a stun is just something magical in the right situation. This also carries over quite well into the late game, as adding an Assault Cuirass after getting Mjolnir and Basher basically lets you shut down single targets. Turning the Basher into an Abyssal is just icing on the Cake. I don't list both because I don't feel I need to, if you get a Basher and it goes on long enough, turn it into an Abyssal. But don't prioritize it, unless you need damage or are out of space.
Once you make a post-Armlet pickup, don't be afraid to sell the Drums. Keep the Hand of Midas until you've basically built all your other items, or it's fairly far along and the gold from selling it will free your inventory and give you the gold for another wish list pickup.
The Orchid may be a "what the hell" on my list, but hear me out. It gives you +30 attack speed, +30 damage, all the mana and mana regen you could ever want, and a 5 second silence that increases damage taken for the target. Which is basically pretty ****ing sweet. It's fairly situational as Lifestealer doesn't need to worry about spells all that much with Rage, but it can be useful versus certain heroes; particularly ones with escapes.
MKB (Monkey King Bar) is basically picked up if you have to deal with heroes that have or will have evasion. It's also not a bad option in general. It provides a boat-load of damage, a bit of attack speed, and the mini-stun which is always good. MKB stacks great with Mjolnir or Assault Cuirass, so those are probably the best pickups if you get an MKB. It also does combine well with Skull Basher/Abyssal so don't fret on that.
I do not have Daedalus on my wish list, as I've never really cared for it on Lifestealer, it does not increase his Lifesteal, and quite honestly can hurt him in the wrong situation. Plus for the money there are far better items, in my opinion.
Lastly is the Heart, always a good pickup on a strength carry. If you are getting good farm, or have 1-2 wish list pickups already, get this. If you are getting a Heart, don't be afraid to sell something to get the space and last bit of gold for it. It will increase damage, mean you leave the Armlet on basically all the time, and are tanky as ****. If you do get a Heart, make sure you get an Assault Cuirass if you already haven't. Adding 1k HP w/ regen and +15 Armor makes you hard as sin to kill.
There really isn't much to say.
Rage is free BKB with attack speed bonus.
Feast is Lifesteal based on the target's total health, which doesn't increase with crits; and why you shouldn't get Daedalus on Lifestealer.
Open Wounds is an awesome slow, and melds great with Phase Boots and Drums for chasing down people, and even escaping in the right (or wrong, depending how you look at it) situation.
Infest is a relatively under-utilized skill on Lifestealer. It can replenish your health, nuke whole armies of creeps (great for nuking swarms of Broodmother droppings btw). It also combos well with a number of heroes for what is basically known as "Naix Bombing".
The Naix Bomb
Basically you jump inside an ally (99.9% of the time, it should be a melee hero), initiate on an enemy hero or team, and you jump out of your buddy doing damage and finding yourself in range for your spells.
Notable Naix Bombers: Spirit Breaker (the best), Riki, anyone w/ Blink Dagger, anyone w/ Shadow Blade, Sand King, Magnus, Nyx Assassin, Anti-Mage, Slardar, just about anyone who can run in and cause trouble on someone.
Also, don't be beyond using creeps to bomb, going from the jungle do behind your forward tower, jumping in the first creep and surprising an unfortunate foe or nuking the enemy creep wave for farm is not a bad idea.
#1. Pick Lifestealer
#2. Follow the guide
#4. Get items
#5. Don't screw up
#6. Repeat steps 3-5 until someone's Ancient goes boom.