If you are a regular supplier of legal services, you must register for PST purposes. This is in addition to your registration for GST purposes.
If you are a member of a law firm, then it is the law firm, or the partnership, as opposed to individual law corporations or partners, that are considered to be providing the legal services. Thus, it is the law firm or partnership that registers for PST purposes.
You can register online through Saskatchewan eTax Services (SETS). You can also register by printing and completing the registration form available online and then mail or fax it to the Ministry of Finance.