I bought the ones from Abstract Ocean and they work great on the 20" rims. Basically, the way you install them is to run them around the circumference of the rim, sticking them as you go. The kit from Abstract Ocean came with more than enough material to cover all 4 20" rims.
BTW, they work great and I highly recommend them. Just be sure to follow the instructions in terms of making sure the rims are stripped of any sealants and are very clean prior to the installation.