I originally made custom presets for portal because the stock ones didn't have the type of sound I was looking for. I wanted more rhythimically quantized presets, as well as ones that stayed in key, that I could use for adding more texture and depth to my melodies. That lead me here, to making my own portal banks, each with over 50 different presets.
Each preset starts with a basic idea, whether it's phase, flange, reverb, delay, distortion or anything else Portal has to offer. I then test it with multiple sounds, and make sure I can actually use it in one of my compositions. Once I have enough presets, I narrow it down and delete the ones that don't match my standards.