This fixture is used for three steps decking, sear, and hammer locations. I recommend drilling the sear and hammer holes first but not required. The main reason is so you have a location to measure from during decking. Pin the frame in the fixture and secure in a vise.
If you have a drill presses use it and secure to the table. Drill one side of the frame at a time with the #22 and #35 drill bits. You can use a cordless drill if you want the ½ bushings won’t let the bit flex or wander. When drilling pull the drill out a few times to remove the chips as you go. Repeat for both holes and both sides. This is the frame after the sear and hammer hole have been added. Now time to measure the known deck height. Hook the edge of the slide catch hole and measure to the top of the frame and write it down. Do the exact same thing to the hammer hole. These numbers will be different but you need to know your starting point for both before you start decking. The goal is to remove the material evenly across the top. So if you remove .015 from the front take .015 off the back. Your final decking dimension to the top edge of the slide catch hole is .348 Mount the frame in the vise for decking.
The fixture is for files only no belt sanders or Dremels. For decking the top and dust cover use a cheep Home Depot 8" file or equivalent. The fixture has hardened rods on the edge that will keep you from filing to low. File across the frame keeping the file as flat as possible. Do the same to the lower section as well. Use a straight edge across the fixture to check on your progress. Make sure the top is as flat as possible and check that you have removed the same amount of material evenly across the frame. This must be done correctly before using the PRC rail cutter. To remove file marks sand upside down on a hard flat surface with 200 then 400 grit sand paper and oil.