How does one become a LEGO® Certified Professional?
First of all, there needs to be an opening for one, which typically means LEGO feels they need more Certified Professionals in a particular region.

The most-recent opening was last November and was presumably filled in December. For this most-recent one, LEGO had a posting on their website where people could apply, and they made a small announcement to fan communities via their ambassador program.

Assuming this is how future certified professionals get selected, basically becoming a certified professional means applying to an opening in the program, and getting accepted (I strongly suspect anything beyond the initial application is protected by an NDA).

Unfortunately, I don't know what specifically was asked from potential applicants to the most-recent opening, but if you are interested in perhaps applying you should be on the lookout for future openings. In the mean time, consider joining some RLUGs or RLOCs and making some connections, to gain a reputation as a builder and perhaps be recommended to apply when there is an opening.