The FAA has written a walkthrough that shows all the administrative steps necessary to complete and finish the 14 CFR 107 licensing process. You will learn about eligibility requirements, how to schedule an official test and other key information.
Sign up for a free account through Drone Deploy and log in to the free Drone Deploy Academy. Scroll down until you see the Part 107 Training Course. Enroll and complete it.
King Schools has a very good custom 107 test creator with 2x the question bank of other free test resources.
Current Part 61 certificate holders seeking a Part 107 Certificate can take the Part 107 Small Unmanned Aircraft Systems Initial (ALC-451) for free on the FAA Safety Team (FAASTeam) website. After passing that, check with the FAA 107 admin walkthrough for the next steps.