So here is my way:
Step 1: Decide on a size. A small tepee might have 4" sides. The one I made has 13" sides. For the small one, the exterior piece and the lining need to be cut at 5" X 10". For the one I made, I cut the two pieces 14" X 28". ( I used 1/2" seams.) Just always make the length twice the width and use a zipper that is at least as long as the short side. (If you use a metal zipper, you are likely to break the needle in your sewing machine - fair warning!)
Extra Step if you are quilting the top: Cut the exterior fabric, the batting, and the muslin about an inch too big. Layer the outside fabric (yellow with green tractors, the batting, and the muslin. Machine quilt the 3 layers together. I just sewed the length of the fabric about every 2 1/2". After quilting, trim this piece (the exterior piece) to 14" X 28" just like the lining.
Step 4: Pull the two tubes of fabric that you've just created, so that the zipper is as centered as you can make it. Make sure you pull the fabric rather taut so that you don't get any puckers.
Optional Step if you are using a long strap handle: Make a one inch wide strap from a 3" strip of fabric using a 1" strip of batting. For my bag, I used a 15" length. Sandwich one end of the strap just in front of the zipper between the layers of the exterior fabric.
Optional Step if you are using a wrist strap: If you want a small handle/strap, make a loop of grosgrain ribbon or something similar and put both ends just in front of the zipper between the layers of the outside fabric. Use a length for a wrist strap or whatever you desire. (This is not shown.)