Here is my process, others probably do it more efficiently. Essentially you are grinding another blade on the knife
1. Drill all holes for pins/bolts, tubes.
2. Mark the tang with how thick you want the taper to be at the butt (I shoot for .030-.040)
3. Like you do for the edge, grind 45 degree bevels to those marks on the end of the butt. This provides a visual reference for where to stop the grind.
4. Use a small wheel (or the idler wheel of a flat platten) to grind out a trough on either side of the tang (centered). This reduces the amount of metal you are grinding and lapping flat.
5. I use a welding magnet and my flat platten. Grind, check, grind again until you get to about 1/4 inch away from where you want the taper to stop.
6. I mark the tang with a sharpi and go back to the platten to finish the grind, this allows me to ensure I am removing material evenly.
7. Lap flat on a surface plate or piece of granite. You can also use a disc sander to ensure the tang is FLAT.
8. For the blade to tang transition, you can blend (carefully) with stones or sandpaper.
Keep in mind I don't grind my bevels until after the tang is tapered and the handle scales are drilled. With some caution you can do it with a ground out blade bevels.