I drill the tang while flat, then taper it, as you've done. That's crucial.
Then I use spring clamps (small C-clamps work, too) to hold one scale to the tang (while the scale is still rectangular), and mount the scale in an inexpensive machinist's vise, so the blade is horizontal both along its length, and across the width. This is all about keeping the centerlines of the blade perpendicular to the drill bit. The scale will obviously be a couple degrees off of horizontal. You can eyeball it but checking with a small level increase confidence. This is important, so that your pins or bolts will end up perpendicular to the centerlines of the blade.
I drill one hole and place a trial pin/bolt through it, to help ensure the scale doesn't shift. I drill any and all remaining holes in the same fashion. Remove that scale, flip the blade and repeat for the other scale.
The angles involved are slight enough that you can trial-pin the scales together and shape the fronts together for a clean match, on a bolsterless design. For a full-tang knife with bolsters, a bit more careful machining/handfitting is sometimes needed. It may sound complicated, but honestly it doesn't take much longer to do it than it does to type out the instructions. No jigs required.
Boom, you're now ready to glue up and pin/bolt your scales, let the adhesive cure, then grind them down to the tang profile and finish the knife.
I feel strongly that a nicely-tapered tang helps a good deal with how your knife balances, and shows a certain skill-level and dedication that helps set handmade knives above the factory stuff. Plus they look really cool!