Click the green flag to start. While in editing mode, you can edit the lines of the pattern by clicking the circles next to the drum names to turn them on/off. In the pattern list, "0" means on and "=" means off. For the circles next to the drum names, red is on and white is off. You can insert and delete lines by clicking the buttons at the bottom of the screen. You can press up/down/w/s to move up or down a line. The "go to line" button lets you change which line you are actually editing. You can change the tempo by clicking the change tempo button. You can click the play button to hear your pattern play in a loop. You can click the stop button to go back to editing mode.
Don't forget to love it! 100% original code. Time to make: around 3 hours Time to update it and make the codes for saving and loading patterns: around an hour and a half Good drum loops can be found on my forum post at http://scratch.mit.edu/discuss/topic/36970/
