Got to play it on my own and sort of poked around with it a bit to see how it works. Slender man never moves, so he doesn't really follow you, he will just spawn somewhere near you. I believe the more notes you get, the closer he will spawn near you. Also sprinting and using the flashlight probably also determines his distance to you. So once you have quite a few notes try not to make sudden turns, as he will never spawn in your vision cone.
After I collected my first note, I started looking around a bunch. I spotted him but he wasn't that close, static will start to fade in if you keep looking at him, but if you look away, turn your flashlight off, or get some distance, the static will fade out. Once I spotted him, I just kept backing away sort of keeping my eye on him and looking away when the static kept increasing. Eventually when you get enough distance, he will despawn. After that I found my second note and spotted him again and did the same thing. There's a bathroom house that I meant to find first since you are basically very vulnerable there since you're in nothing but narrow halls. So I sprinted through the house searching as quick as I could... yea, bad idea, turn around a corner and he was right there, I jumped in fear. Damn this game.