READ ALL STEPS BEFORE EVEN BEGINNING!!!
Step 1: What you'll need
-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...
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.