Mark,
I first shadowed an official measurer and assisted him on a couple of scoring sessions. Then, if you go to one of the clubs websites, you can request an application to become a scorer. Then they invite a certain number of applicants based on experience, and location. I have heard it is fairly tough to get an invitation, in my case there are only a handful of measurer's in my state and I am the only one for miles, so I got an invitation almost immediately.
Then, you have to go to a class, for P&Y it is 3 days?? and for B&C it is a week long adventure.
One thing to keep in mind, B&C will not accept P&Y measurers scores, whereas P&Y will accept B&C scorers... so in my case, I am official for B&C but not for P&Y, even though I measure more animals for P&Y.
Good luck on your quest, it is a volunteer position and it takes a lot of time... but is rewarding and you get to put your hands on some true bruisers!!
adam