An important note: you are merely breaking the gloss finish here, rather than sanding to bare wood or even removing a significant amount of finish. If done correctly, the neck could be buffed back to a gloss with the remaining finish, so you're not really doing something as permanent as it sounds.
^^ This...
For me, a sequence of grits from 600 to 2000. Auto parts stores will have a pack of assorted high grits that they sell for auto body touchup work. Water with a drop of soap won't hurt, but isn't strictly necessary. Fold the paper into thirds or fourths and conform it to the shape of the neck. Sand LIGHTLY with the grain (up and down the neck). You're just barely taking off some gloss, not trying to strip the neck.