I used a fleece blanket I found on the street. An airline blanket would work fine, or any fleece garment that doesn't fit you.
There are a few tricks to this project:
Make a paper pattern so you can refer to the pattern and evolve your next pairs of socks to perfection.
Fleece stretches more in one direction than the other. Put the stretchy direction around your foot.
Stretch the cloth while doing zigzag sewing - then the seams will stretch with the cloth later.
Sew the zigzag seam so the needle overshoots the edge. That binds the edge.
If your thread tension is low you can pull the seam around so it looks whipstitched butt-to-butt and lies flat.
Step 1: Stretch Test
This particular blanket stretches most in the direction of the stripes.
So I'll make my socks with the stripes around the legs.
Step 2: Make Paper Patterns
There's a sole, a back, and a "vamp" which is the long part that covers the instep of the foot.
The size of the cloth pieces will just match the diameter of each part of the foot they cover.
No seam allowance is necessary.
Step 3: Mark the Cloth
Use a sharpie marker or anything else.
Permanent is okay, the lines won't be visible on the finished socks.
Step 4: Cut Out the Pieces
The fleece sticks together nicely, so if you're confident you can cut out two or more layers at once.