Step 1: What You Need
Step 2: What You Do...
Lay the seam pieces together, keeping ends more or less even. A long piece can hang over at both ends. Take up the ends and tie a knot in the middle. To make the knot tight enough to hold up to a dog you need to pull them all together and then pull each piece separately at both ends. This will make the knot nice and tight. This keeps the toy together AND gives a nice semi-hard chew ball for the dog (nice for teething puppies). It also gives it a little mass so you can throw it.
I've made these out of fleece scraps as well. Keep in mind that the fleece versions should be for dogs who SPIT OUT the pieces because the fleece will eventually pull apart. Our dog does fine with the fleece.
Since it's denim, you can wash it if you want. I'd say use the gentle cycle since you don't want the knot to get worked out. After a wash or two in the gentle cycle you can probably use the regular cycle. Or, if you've got one, throw it in the pool once in a while.