Brain-bending. I couldn't tell you now the exact details of how it works as I've used MIDI to program 303s ever since Nigel put a CV/Gate/Cutoff/Accent input on his in 1995, and even when I was programming it every day I still had to have the manual nearby. It's similar to the step-sequencer in many other devices (SH-101 or Sequential SixTrak for example) in that you get very little feedback of what you're doing and have to hold the whole thing in your head whilst your programming it. Basics: you step through the pattern (max 16 steps) one step at a time, programming a note of the required pitch or a rest into each one. Each step also has options for octave shift, tie, slide and accent. You can chain patterns together to make longer melodies, and make songs out of chains and patterns. If once you've programmed your riff you then realise you've made a mistake (which is fairly probable) and want to correct it (who knows, the mistake likely sounded better than what you intended) you have to listen to your riff a million times, counting 16ths like crazy until you're as sure as you can be that you know where in the riff the mistake was, then step through the pattern and correct the note(s), praying to god you didn't get confused by the fact that the slide function applies to the note AFTER the one you programmed it on... oh you did get confused? No matter, your riff now has a quirky syncopation in it! Cool! And oh look, the 'C' key double-presses sometimes so you just overwrote another step with the wrong note; no matter, put an accent on both of them, crank up the res and the accent level and listen to that baby squeal!
For those of you after ultimate reality in your 303 riffs, this page is the Bible (at least for me); geeky maybe but it really does make a difference when used with a good 303 emulator.
[fanboy rant]
If the 303 is one of the most important instruments in dance music, then Phoscyon is perhaps one of the most important plugins. Admittedly it's been a while since I had my hands on an analog TB303 so my memory of the sound is quite old but it really is that good to my ears. I'm in heaven here right now.
Raw instrument output - no plugins or processing
[/fanboy rant]