This is my first instructable.
Step 1: What You Need
A tennis ball
Basic knot tying skills
Step 2: Set Up
Put the ball in the sock, and push it to the bottom. You could leave it in the middle and tie knots either side, but my dogs prefer it at the end :)
Step 3: Knot Tying
This is a very simple knot, one of the first everyone learns. Twist the end with the ball, and loop the other end round and under. Then put the end through the hole you should have, and pull the knot tight.
That is it!! Hope you and your dogs enjoy :)