As already stated Sailrite and most other marine supply outlets have repair kits with the thread, wax, palm, needles and some even have a small amount of sail material. I think Sailrite even had a book for about 12 bucks on DIY sail repairs. I had a fairly good sized area (about 2' x 6") that needed repair on a very old sail with several small rips and a small amount of UV damaged material. I bought some dacron sail material and lamented both sides holding the repair material in place with carpet tape on a flat surface to keep them lined up and smooth. Then I just followed the original sewing pattern. The area I repaired was in a reinforced and heavily stressed area of the sail so I probably went over board to make sure it lasts until I can afford to replace it.
1976 Chrysler 22 "CinJam"