Depending on your RDO, the process changes. But, in general, you have to do the following:
- Transfer your RDO (especially if you're transitioning from full time employment)
- Fill out a Form 1901
- 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.