The way a 2D level works is this: You make the level as normal. How ever, when you set 2D in the map header (see header guide on SRB2 World) the camera starts, I think 128 units south from the player start, and you can only move left and right. That's why it just looks like a bunch of sectors all in a row. To make one, you just make the level normally. However, with exceptions, you will have to work on just one line of sectors and stuff. Like they said, look at the existiing 2D maps. I think there was a 2D match map lying around somewhere, and you can look at that too. Just expect to use a lot of FOFs and other stuff, and it helps to plan out your map before so you know what you are doing.