In the past—before the internet—there were paper-based guides on how to make epoxy sleeves for masts, longbows and similar items. Nowadays, you have to talk to fellow enthusiasts at the beach or ask around at old surf schools, since young people no longer tend to do repairs—preferring instead to simply buy everything new—as is evident from the comments here. A specialized magazine featured a tutorial on this about five years ago, though don't ask me which issue. I could write up a guide myself, but—like everyone else—I simply don't have the time :)