Welcome to the forum. :waving:
It sounds like you'll need to pick up stitches along the sides and cast on edge of yhour bag bottom. If you go to the basic techniques tab at the top of the page, in the And More section there's a video on picking up stitches.
There's also information on knitting in the round if you need that.
Hope to see a picture of a FO soon!
Happy Knitting xxx