Start with dead squirrels as soon as you get it. As it gets older get it around as many squirrels as possible. Caged work if you can catch them or just take it to the local park with lots of pet squirrels. Biggest thing is make sure it knows what you want it to tree and spend as much time as possible after 5-6 months old with it around squirrels. A good tree dog doesn’t need much training it’s natural. When I picked my two there were 15-20 in two litters different gyps same male. The two I wound up with were the only two that stayed after the squirrel I had pretty much the whole time.