This guide shows how I personally play swain as my main and favourite character. The great difference with the other swain's guides is in the early game and the VERY expensive stuff I build. I prefer playing a barrier than an ignit. Everybody knows swain isn't an early game character so if your opponent plays very safely you usually won't take a kill in 1v1 during laning. Most important, swain isn't bad on laning, and is feared for his late game, so as I do myself, you'll probably be ganked a lot ; I haven't counted how many times barrier saved my life! Also I prefer barrier than ignit in late game which I find much more in synergy with the champion.

As I said, this stuff is expensive and you'll struggle to reach to your core items. But that doran's shield that I personally play when on "bad" matchups allows you to have a safe early game and hold a very good laning.

Your game will probably end before you finish your whole stuff but keep in mind swain will be brillant in mid game, and a pure monster in late.