Made out of denim scraps from my mom hemming my little brother's pant cuffs. Very simple, but the difficulty depends on what material you use. It might be simpler just to use a pant leg, but we didn't have one laying around or anything.
READ ALL STEPS BEFORE EVEN BEGINNING!!!
Step 1: What You'll Need
-Some sort of fabric, cloth, material, etc., including leather/suede.
-The appropriate sewing stuffs (such as needle, thread, scissors, and superglue if you're bad at tying knots)
-A water bottle (to check for size. Preferably full when testing for strength)
-A well-thought-out plan (or yours will end up BADLY!)
Step 2: Step 1: to Start With...
Get a long rectangle of material. It should cover the cylinder-shaped part of the bottle, the part you won't be grabbing. Call this "A".
Step 3: Step 2: a SECOND PART!!!
Get another rectangle of material, slightly shorter than A. Call it "B". If you were to stick one end of A and one end of B together so that they overlap (overlap them in the way that you will sew them together later). The other end of A should be at the same place as the other end of B, and you should be able to put a water bottle between them with ease. If not, get a longer piece of material and replace "B" with it. Call this longer piece of material "B".
Sew these together at one end and proceed to the next step.
Step 4: Step 3: C and D.
Now your water bottle holder is really taking shape. Get two strips (or else one very large piece) of material and label them C and D. They will be the sides of your water bottle holder. Fasten them as you like, but they have to allow room for the water bottle, so be careful and measure it.
Step 5: Step 4: E.
Possibly the hardest one: the belt loop, and it has to work around C and D. Attach it to the back of A by its shorter sides, which should be the top and bottom sides. Mine straddles C, but hey: you might have done it differently.
Step 6: Step 5: Try It Out
Tah-dah! You're done!