If the tails are still in, then cut the skin along the backside (brown side) rather than of the white side. e.g. don't cut the white side in half.
After that, I simply tack the stretched skin/tail on a board and pour a liberal amount of salt covering the whole thing. Let this dry outside for a week or so and then remove any grease/fat with a shrp knife. This is the easy way, but it leave the tail very "hard", not tanned.
Theer are more complicated ways (tanning) and I don't use them unless I am doing 6-8 tails at once.
Pete AKA Frenchcreek from Calgary