I use a large Dunlop thumbpick which is just slightly too small for my thumb and in order to get a good fit I plunge it in very hot water for a few seconds (don't leave it in too long or the thing will uncurl), place it on my thumb and immediately put it under cold water still seated on my thumb. Then with various grits of sand paper made fast to a board I shape the picking point to my liking ( I like it to be as short as possible). Next I take a fine file and remove plastic from the area above and opposite the picking surface until I get the action where I like it and then smooth it with fine grit sand paper (don't remove too much material). Finally I add a bevel on the picking surface where is strikes the strings. It takes a bit of trial and error to get it the way you want it so don't work too fast and keep trying it as you go.