So you just got some amazing new drum samples and you are about to build a custom kit on your SPD-sx, but you have no idea WHERE to put each sound. It probably won't help if I tell you that AFTER you choose 9 sounds for the kit there are STILL 362,880 combinations of how to lay them out! Yikes... well let's narrow down those options so you can get to playing your new sounds.
The layout of your drum pad really depends on what sounds your are using, how your are using them, and if you are also triggering tracks or other external instruments. First let's make sure we are on the same page with how the pads are numbered. Roland numbers their pads 1 through 9 starting at the top left and ending at the bottom right (see image). The top three pads are smaller and rounded making them a little more difficult to trigger at specific dynamics. Because of this limitation I like to put single dynamic samples or trigger tracks/click and other MIDI controlled devices on pads 1, 2, and 3. Now, if you are able to pick 3 of your sounds and assign them to the top row we are already down to a manageable 720 permutations for pads 4 through 9! Not bad... but we can make it a little easier than that.
For the bottom six pads consistency is key. I strive to place similar sounds on the same pad every time I build a new kit. This means, for example, that every primary kick is on pad 7, primary snare is on 8, and more "auxiliary" sounds like toms will be on pads 4, 5 and 6. Pad 9 will generally have another "back-beat" sound like another snare or clap, but I might put a high-hat or shaker sound instead. Like most drummers I play snare with my left hand and ride/hats with my right, so laying out the SPD-SX this way gives it a familiar feel and coordination to playing an acoustic kit.