How do I register to the BIR as a Freelancer?

Depending on your RDO, the process changes. But, in general, you have to do the following:

1. Transfer your RDO (especially if you're transitioning from full time employment)
2. Fill out a Form 1901
3. Register at your RDO

A more comprehensive blog post regarding this topic was written by Ginger Arboleda:

Freelancers’ Questions About Taxation, Answered!

Highly recommended! Please head on over to her blog to understand how to register.