The rule of thumb is that you only cut 1 foot a year off the top and they spray it with a wilt protector that a nursery can recommend. You can only due this 6 weeks before frost. Or in a warmer climate you can cut anytime.
This is a type of tree that will grow no matter what and it is difficult to kill it off. You can use an electric or gas powered hegde trimmer to shape it in the way you would like it to look. You should start with a guide so that this is a managable project.
