Complete all chapters on Veteran.

Veteran difficulty is available from the start and it would be a good idea to start there since the difficulties do stack. Starting here will give you a decent idea of what to expect on your Professional playthrough.

If you have a friend who has already unlocked Professional, you can skip this step and begin there, but you won't have access to any guns with unlimited ammo, which is extremely helpful on your Pro playthrough. You can ride on your friend's coattails though.

hoped this helped
-Toes