If the cover is Acrylic Canvas, the stick-on dacron letters won't stick and will need sewing in position. You can also buy PVC lettering which you also have to sew on, from Kayospruce. If you go this route, use double sided tape to position the letters before sewing them, (the double sided tape also from Kayospruce)
Chandlers here have sail repair tape. About 40mm wide sticky back in white or other colours. This could be split length ways for narrower tape. Once stuck on sew it with domestic sewing machine. olewill