The first step is to learn how to accurately grade comics on your own. The next step is to learn when a press can help, when it can hurt, and when it won't make a difference.
Some people press everything. I don't agree with this approach. Lots of books are being pressed when there is no need to do so.
In short, I try my best not to expose my book to moisture, pressure, or heat...all of which are used, to varying degrees, when a book is pressed. You also risk all sorts of damage, like popped staples and split spines.
That being said, there are instances where pressing a book will be very profitable. It can also help out desperate people who play the registry 9.8 game, of which I was a former major player.
Pressing is NOT a miraculous method. The learning curve can be steep and I wasted a lot of money before I became more judicious about improving my grading ability and building an understanding of how and when you should press a book.
If you blindly press everything you submit, you will lose a lot of time and money. Learn how to accurately grade. Learn how and when pressing can help.