Begin with a piece of felt twice the length of your crochet bag (felt can be bought by the yard at most fabric stores). Lay it on a flat surface, folded once for double thickness.
Using the bag as a template, cut the felt around the bag so that it is the same size and shape as the bag. Since the bottom of the bag was placed at the fold of the felt, no sewing will be required for the bottom of the lining.
Sew up the sides of the felt lining with a simple whipstitch and matching thread.
...then turn the whole thing inside out.
Whipstitch the top edge of the lining to the bag, making small stitches. The smaller the stitches, the neater the finish will look.
Turn the bag right side out and admire its beauty.