Registration for PBP 2019 will be in several phases. The first phase will be pre-registration. During pre-registration, you can reserve a spot in PBP 2019 even though you have not finished qualifying yet. When you can pre-register is determined by the longest ACP event you completed in 2018. The schedule is spelled out in this document. Final registration begins on May 25th, 2019 and closes on July 3, 2019. Important: (from the PBP 2019 website) "You will have to convert your preregistration to a final registration before 20 June 2019 at midnight (French time) to preserve your preregistration."
Regardless of when you pre-register or register, the first step that must be completed is to create an account with the ACP for your registration. This step can be completed now (as of Monday, January 7, 2019). To create an account, go here and click on the "Register now" button on that page, or go here: https://inscription.paris-brest-paris.org/ Once there, you have two options, one to create an account, and the other to access an existing account. Keep this link handy as you can go back here later to check on your registration.
Once you create an account you can add contact information for the ACP to reach you, and save a little time when your time for pre-registration comes up. Once you have accessed your account, you will see these options: registration (not active until at least 1/14/2019), results, and 'my account'. If you have filled out the contact information when you created the account, your results page may have all of your 2018 ACP results. It is expected that your 2019 results will also appear and from which you would select the ACP brevets and certificate numbers to use for qualifying.