If your paint can is really old, this step can be very easy, as old dried paint often peels off in large pieces. I used a scouring pad for the dried-on bits.
This next step is the fun bit. You can pick whatever you like as decoration for your paint can planter and use Mod Podge to affix it to your paint can.
The last step is to apply a finishing varnish or gloss to seal and cover your work. The pot will inevitably get wet when being watered, so you want to make sure your decoration is protected.