Being lazy... I didn't measure my bottle, but simply wrapped the fabric around the bottle until it was about 3 inches overlapped. I then cut there and figured out my height! I also cut on the fold so that I would have one less side to sew!
I then cut some thin batting to the same size for some sort of insulation and stuck it inside.
I used some left over bias tape and pinned it to the outside two edges. I would have done all three, but I didn't have enough!
After sewing that I found a Yo-Yo and matching button for my clasp "hook".
I flipped it over and on the other side I folded it in about half an inch and sewed that down.
Then measured a piece of dowel to stick inside so that it wouldn't collapse here. (You will see what I mean when you see how I got it to close.)
I had some cute frilly elastic like ribbon. I did one short and one long in case I ever used a larger bottle. If I had velcro I would have used it, but.... I didn't!
I used some puffy paint to write her name on the outside. I put it over the bottle to keep it straight! And I was done!
Here is the clasp portion. I put both the long and short elastic over it so it wouldn't dangle! Pretty sure she now has the cutest "snack" water ever!