I replaced my blower motor with the same size as the one before and I can not get the new unit from shaking. Any ideas

The fan is probably out of balance or the mounting bracket is defective.

How do I balance the fan?

You do not balance the fan.
It is best just to replace it.

Whenever a motor job is done the fan has a good chance of being damaged.

Now if you clean the dirt out of the air scoops it may spin better.