First you must know how to crochet, and sew. There are patterns online that you can use if you already know how to crochet. You can modify the pattern to make it your own once you are comfortable with it. You can also look at existing fly bonnets that you like and mimic the pattern there if you are able to identify the stitches used. There are some patterns that use knitting as the method instead of crochet if you would rather knit.