Hi,
You need to receive a logbook endorsement from an instructor to sit for the FAA CPL knowledge exam.
You may decide to do an online ground school or do a home study course for the written exam and be signed off by an instructor to write the exam. There are many oral exam guides available in the market to prepare for the exam.
I would suggest you buy Gleim.
If you need an endorsement for writing the exam, PM me. I have a CFI (FAA) and an Advanced ground Instructor (FAA) certificate.
Hope this helps.