This could be made using many kinds of rope, I chose to use a crocheted Romanian Lace Cord
Step 1: Locate a rope, or crochet a cord, length depends on size you want. These are the size results I came up with:
24 inch cord creates a collar which adjusts from 9 inches to 16 inches
30 inch cord creates a collar which adjusts from 13 inches to 21 inches
36 inch cord creates a collar which adjusts from 15 inches to 26 inches
..that ought to cover most dogs.
If you use a crocheted cord, leave long ends to use for sewing later.
Step 2: Tie the adjustable knots. Tighten them so that there is a little resistance to them sliding - you don't want it to adjust too easily.
Step 3: Sew a ring onto one end.
Step 4: Sew thread through knot back and forth to secure (so it doesn't come untied) - being careful to NOT sew the knot to the rope disabling it from sliding!
Step 5: Repeat sewing for other knot... and that's it!
ps: this could also be made for cats..