1. FIrst and foremost, become an active member of NWMA. To race PHRF in the Northwest, one must be a member of a participating yacht club.  NWMA is a very active member club.
2. Obtain an official sail number from PIYA (Northwest).  PIYA is the US issuing body for US Sailing.   The number is regional but applies to all the US/Canada.   It stays with the boat and avoids the likelihood of multiple boats with the same number (which still occurs occasionally).
   PIYA is also the Safety authority that governs just what safety equipment is required for the various classes of races.   Each yacht club joins PIYA and may or may not have voting representation on the board. The link above requires that an email be sent to jan@piyasailing.com requesting an official number. I already had number on my sail and found out that this number doesn't actually have to be on the sail to race but if it can be, it will avoid confusion as it will always be unique.  
3. Download the PHRF rating form.  This will give instructions on how to get through step 4.  PHRF-NW is the organizing authority here in the NW.   It keeps track of the 2000-3000+ race boats in the region, assigns ratings, etc.   Virtually all the individual yacht clubs are members and conduct their events using PHRF ratings to provide a level playing venue. 
4. Use the form's instructions to measure your sails.  The multihull handicapper takes this data which drive a series of formulae and ultimately generates a rating that reflects the potential performance of your boat.
5. Send a copy of the completed rating application form and a check to PHRF in Gig Harbor at this link.  The cost is around $55 per year.  They will then send the form along to our handicapper.  Give him a couple weeks and he will get back to you with your number or with any questions he may have.  If you don't hear from him, let me know and I will put you in contact with him.

A bigger question is if they are required to be displayed on the main, spin, and any overlapping head sail.  Most serious races require it usually in the sailing instructions and some like Swiftsure and Van Isle (Technically offshore race number criteria) require them also on lifelines or like Moxie, on the hull (it also looks cool).  Some local club races will wave the displayed numbers but I found it the exception.

Most people don't know but PHRF is all class racing.   We race in the PHRF Class under the auspices of US Sailing.     Also, Multihulls are rated independently of mono hulls and the numbers don't correlate. Moxie is rated 0 but it is not the same as a mono rated 0.  All perfectly clear, right?

Finally, PHRF Ratings are not in either a chart nor are they plucked out of thin air.   They are our best guess of your boat's potential performance based on all the information that you submit on your application.    The rating is also an OBSERVED Performance rating.  For example, Scooter was a F-25A that should have rated a bit faster than an F242...say around 100.  Maiden voyage it corrected in first place at a major regatta.   Re-rated to 90...1st place...re-rated to 80...1st...  I am not sure where it ended up.  The reason it kept changing was because it was a perfect boat and sailed it to its performance.

Also, it is the skipper's responsibility to inform the handicapper of any changes...modifications, different sails, taller rig, different boards, etc.   just because you have got a rating, it doesn't mean you can go hot rod it without being re-rated.   We also don't buy the "well it has all stock sails".  You will need to measure them and there are plenty of us who will come help (we'll need your help sometime too).