Where you plant would depend on what kind of hydrangea this is. It looks like a mac. Most do best with some dappled shade but I would research which is best to be sure.
I wouldn't plant this now during the dog days of summer. You may be successful but I've never been. It's hard to keep anything hydrated duing the intense heat. Best to wait until the weather cools a bit. Not sure when that is for you but for me in zone 7a, it is early September.
How depends on your soil. But it is safe to say that a hole with some organic matter thrown in, a nice layer of mulch and a good soaking followed by frequent waterings until frost should keep it safe through winter.
Good Luck, it looks like you took good care of it until now.