Step 2: Tap the holes
This is where you really have to take your time. First, grind the point off of the tap. Don't grind into the threads, just take the point off.
Here is the original tap:
Here is the tap with the point ground off:
Now you can start your threads in both holes. Be certain you have the tap straight and square in the hole, and gently start cutting your threads. Remember that about every half turn to turn you thread, you need to back out enough to clear the shavings(about a 1/3 to 1/2 turn back). Just keep going back and forth until you bottom out the tap in the hole. Do this to both holes.
Now grind the tap until two of the tapered threads are gone so you can continue to tap deeper. Once again, gently cut threads in both holes until you bottom the tap out.
Now grind a couple more threads, and repeat the previous steps to get the threads deeper.
Finally, completely grind of the tapered section of the tap, and finish tapping both holes. You should have clean threads almost to the bottom of the holes, if not to the bottom.
No taper left on the tap:
Step 3: Trimming and installing the Heli-Coils
You must trim the Heli-Coils to length before you install them. Use a pair of wire cutters and cut the Heli-Coils down to 3-4 coils/threads. Watch your eyes when you do this as the cut coils tend to go flying. Now the Heli-Coils are ready to install. Use the installation tool to thread the coils into the threads you tapped. The tip of the coil should be at least a half turn deep inside the hole. There can be no excess sticking out of the hole(because it needs to slide into the power-tube). And the tip of the coil helps keep the coil from coming out with the screw.
This is how much mine needed trimmed(bottom of coil to the left):
Run some #10-32 screws into the Heli-Coils and tighten them down to be sure the Heli-Coil is seated, and you are done!
Now you can put your marker back together without worrying about stripping those soft aluminum threads anymore.