Not sure if this is the "right" way, but this is how I do it.
First I cut the scales to the length I want them, then I tack one onto the tang with a dab of superglue. I then outline my tang with a pen or pencil against the inside of the scale. Carefully drill the holes for pins, and repeat process for the other scale.
I then temporarily pin them together, and use a bandsaw to cut to the outside of the line I have drawn, only leaving a small amount of wood to remove once glued up. Then I will round off and finish the front of the scales to whichever grit or polish I will be taking the scales to, as once they are glued on, it is near impossible to get that area sanded without scratching the blade.
I proceed by protecting the blade with several layers of tape, particularly infront of the ricasso, as thats where your sander or file will most likely make contact if you were to slip up. Then glue and clamp with the blade pointing up to prevent epoxy running down the blade if there is an excess.(I always put a block of wood that I have mortised a slot into on the tip of the blade for safety. Even though the blade is taped up, one slip up could result in a nasty injury that could have been avoided.) Then I continue with the final shaping and finishing of the scales.
Hope I helped in some way. Good luck, and we want to see some pictures once you are finished.
Brandon