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 2: Line up the short edges of the fabric right sides together with the zipper sandwiched in between. Whichever fabric you want for the exterior of the bag should be on the TOP side of the zipper. Sew these 3 together, as close as you can get to the zipper teeth, and as straight as you can.
If the zipper is too long, let the end with the zipper pull hang over the side.
It looks like this after you sew the seam:
Step 3: Line up the other short edges of the fabric with the other side of the zipper and sew.
When you are done, it looks like this on the outside:
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.
Sew across the bottom. I reinforced the seam over the zipper and the handle a couple of times.
Step 5: UNZIP THE ZIPPER. You will be sorry if you don't!
Step 6: Pull the two layers of fabric in the other direction, centering the zipper between the two sides.
Optional Step if you are using a long strap handle: Insert the loose end of the strap just in front of the zipper between the layers of the exterior fabric being careful not to twist the strap.
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.)
Optional Step if you are using a long strap handle: Insert the loose end of the strap just in front of the zipper between the layers of the exterior fabric being careful not to twist the strap.
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.)
Sew this seam, over the zipper, stopping two to three inches from the end of the lining fabric. Reinforce the seam over the zipper and the handle AFTER you are sure the handle is OK.