First, they do NOT make "small bonsai." Ramification is difficult at best, so a 2-3 foot tree is a must for his species. To get backbudding, you let a branch grow until there are 4-5 leaves. Cut back to one leaf. Do that will all branches.
In the winter, you must remove the large end buds off every branch. This triggers small, vestigial buds just behind the large bud and back down the branch.
Then, roots must be pruned every year for the first 5-6 years or more because the root system on a sweetgum is as coarse as the branching up top and if you don't get ramification in both places you won't be successful with this species.
Luckily, you live where these may be the most common tree around so you should have a number to work on.
I think Brent has an article on sweetgum and his end-bud-removal technique. He's the one who taught it to me some years ago. www.evergreengardenworks..com