Cut the legs off.
Sew along the bottom and then don't sew the corner like this.........
If you do then you can cut it off and then turn it and sew the corner from the other angle. This gives it a boxy bottom. Trim the excess.
Then cut a long strip of jean leg for the handle to whatever length you want. Attach it to the inside of the sides of your bag.
Take whatever fabric you want and hand measure how wide it needs to be to fit inside the bag. I little wider is better then a little shorter. I had one that ended up with about an extra inch of fabric. Sew yourself a casing on top for the clothesline....or whatever you use for the rope to close.
Sew the fabric inside the top edge of the bag. Make sure that when you flip the fabric up it will have the right side facing out. I started in the middle back and then if I have extra fabric left over I just gather it there.
Don't do what I did and make the fabric too long. If you do you can trim it and make another pocket for your rope.
I then flipped the fabric out and top stitched it.
It gave it a more finished look inside. I didn't line the jean area.
I simply cut a slit in the casing and added the clothesline. I guess you could do it the right way and not sew your fabric all the way up and leave an opening for the robe to go. However, I am lazy and just cut a slit....in all the bags.
Super simple, super cheap bag made in 30 minutes. (I used clothesline because it was the cheapest option)