I think I was about 16 or 17 years old.
Got a sewing machine and some sort of reasonable heavy cloth or if you are lucky, some sail cloth that has been discarded, measure from near top, at least 2/3 of the mast and then attach to the base and then out to the end of the boom. You dont need any more than that. Realize, it is a 'flat sail'. Attach the sail at 3 points using grommets. After you get all this finished you might want to strengthen the sail additional cloth with some additional cloth along the edges and 2 or 3 pieces from the mast outward to the edge of the sail.
The most expensive part is buying the sewing machine, if necessary. Maybe one can rent a sewing machine?
Here's your chance to be creative and artistic at the same time. No says it has to be all one color. LOL
I have been known to replace a small panel near the peak of the sail, with a white cotton dish towel and iron on tape on the towel and additional stitching securing the iron on tape. Worked fine. Of course I did get teased from my dad (his boat) and his friend.
It was a fun mid winter project that turned out well.