This is my first build so be easy on me ;)
(Title credit to Damatari ;D)

i made the guys for people who want to learn how to play swain
in my build you will find...
- Runes
- Skills/Combos
- Summoner skills (Duh!)
- items
- and how i lane with him

Runes
So i went with alot of magic pen.
magic penatration is amazing with swain because of the fact that your lazer bird will do alot of damage especially early game when you are Harrassing with him to get that ever so wanted first blood and late game it just gets even better. I also ran some Cooldown reduction on him because sometimes when you just get out of your ultimate, an enemy pops out of nowhere to get that last hit on you well with these runes that won't be an issue with swains life steal! >:) isn't swain just a little bugger? :P.

Summoner Spells
I ran Ignite and teleport on him, i use these because you can get into lanes if you need to go back early game and help with a gank with teleport and ignite just incase you need that extra boost in here and there or an enemy is trying to run away from your awesomeness, YOU CAN NOT ESCAPE SWAININATION!!!.

For items i just went with mostly ap with some magic resitance reduction and some spellvamp. The spellvamp will go well with your Ultimate and the magic resitance helps with your magic penitration and normal abilities.

Passive - Carrion Swarm, this is your passive and whenever swain skills something he gains back his mana, useful for when you are out on the lane with alot of health but no mana for harrassing.

Q - Decrepeify, This is your first skill and the one you will be using the most, it does a damage over time like most of swains skills and slows your enemies making it easier to get off your snare.

W - Nevermore, This is your snare, your snare stops the enemies and makes it easier to get Torment and ignite and possible another Decreify off

E - Torment, This is one of your more high damage Damage over time skills, it will get in that last hit most of the time along with ignite it is very very deadly.

R - Ravenous flock, This is your ulti and thing that will most likely save your life when in teamfights or some of your enemies attempt to get a bit cocky and take you own alone. you shoot out three crows randomly and the crows come back and heal you yay for lifesteal!!! -note of warning, this drains your mana quickly activate it when necessary, but use it often you can use this and nevermore to farm creeps for quick mana-

Your Skill Combination!
Q->w->E->Ignite->R= Domination!

Ok so in the beginning you will want middle, if you want to take a sidelane you can but it might be a tad difficult not to Killsteal the person you are laning with. When you are in the mid lane, Harrass with you first skill as much as possible to keep him at bay, once you hit level two that's when you can start to snare, snare use your lazer bird then ignite if they aren't dead and you are level three, use torment, they should drop dead by this point. So you are well on your way to swainination it is mid game and you most likely have your mejai's and probably have your hextex revolver, good well now you will be of more use to your team, snare then go bird form and do your reagular combo, if they are ganging on your teammates from the beginning of the fight use your ulti and snare them so your team has a chance to run away.
Thank you for using my build comment and rate please!!!