The numbers are already reserved for your account when they arrive. You activate each number by making a journal entry for the book; first, you'll be prompted to enter the details such as title and author. A simple way to start the activation is by entering the BCID into the box at the top of the page. That's pretty much it!
They are easy to use. I use almost all pre-numbered labels.
You just have to remember to go to Journal Entry instead of Register Book. ("Registering the book" will give it a new number, not the one on your label, so make a journal entry instead, as Moem explained.)
I have a whole lot of stored pre registered BCID that I use when register a new book. I hand write the number on the label (no printer) I have not done this for awhile and now I have no idea where the numbers are..