Every single level is hand drawn on a piece of paper. It allows me to alter it a little bit if needed. Here's a picture of one of the new levels:
As you can see it's just a simple layout, not a masterpiece. However, it is deadly useful. I often draw numbers on walls, they correspond to each portal I have to place in game, in order to finish level. Sometimes it's really helpful, especially when I'm trying to complete level I've designed few days back and I have completely no idea how to solve it. At least I know it's a good level!
Here's how the same level looks in game (rescaled Retina version - not so crispy):