Make the bottom (the flat circle) until it is the same size as the bottom of your water bottle. If your bottle is an odd shape or gets thick and thin, then measure around the widest part of your bottle. Make the bottom until the circumference (around the edge of the circle) is the same size, give or take 1/2 inch, as the widest part of your bottle. Then move onto the sides. You don't have to worry about the math, each additional round of the bottom adds one more repeat onto the sides, so you just work each side round until you get all the way around. Easy-peasy! As for the height, be sure to fill your water bottle and put it in the holder, then lift from the top of the holder to see if the height is good. A full water bottle will stretch the holder so it's easy to make it too tall if you don't try it out with a full water bottle.