I have been making a few recently -- starting with a blank preset and adding what elements I want and sculpting each sound. It's a lot of work, but really no different than doing it in the DAW with individual samples in a track. To get started though, start with a preset you like and try modifying the sound in some basic ways -- adding reverb to the snare or changing the envelope to gate it, or shortening the kick and trying to add some mid-frequency presence.