This is from the tinyloops website, I picked it up via http://www.dinsync.info/ blog

How to programme, play a Roland TB303 in track and pattern mode.
Nicely broken down into steps, all clearly explained with picture.

http://www.tinyloops.com/tb303/pattern_write.html

I did a basic video of how to program a TB303 which covers some of this material but not as in depth.

Happy acid making.