Look at the willow twig on the left.
Pick out a willow or maple twig in the Springtime or Early Summer time when the sap is flowing best. Try to find a piece that is straight and is free of knots. Silently ask the tree for permission to take a twig. If you are given permission cut your twig and thank the tree.
I think trees are happy when you want to use their wood to make a musical instrument instead of for the usual hot dog stick or firewood.
I always use willow instead of maple because willow are more plentiful on my property and they grow back very quickly when cut. My grandfather prefers maple because it is straighter and has less knots. Maple also grows back quickly if cut from a large stump.
Cut your twig to a length of 6 to 10 inches, the length is not real important, you just want a big enough piece to work with. You want it to be a little bigger around than an average adults thumb but no more than about 3/4 of an inch across. If it's too big around you will have more trouble slipping the bark off.