This great how to was provided by - http://www.instructables.com/id/Easy-amp-Cheap-Dog-Toy/
Step 1: Ingredients
Tennis balls work well since the felt keeps the rubber from splitting.
Take a walk around any public tennis court and you're bound to find some used balls under trees, behind buildings etc.
Treats - smelly and crunchy.
Your favorite cutting tool for working with rubber.
Step 2: Make the Cuts
There are numerous ways to make this toy harder or easier depending on your dogs determination, it's likelihood of destroying the ball, and the size of your treats.
Making a cut along the seam leaves a flap that can be tucked in for added difficulty.
Making a straight cut can lead to more crushed treats instead of "released" treats.
Cross cuts required only one or two chews before the treats fell out, but have an advantage in that they can release the treat when battled around.
Cutting only the felt and creating a deep pocket creates a good space for kibble to fit. Our dog tends to just rip the felt off.
We use the Cross Cut and the Seam cut most often.
Step 3: Load and Go
Load 'em up and watch 'em go!