It's a straightforward job, but requires the right tools in good condition. A reamer is preferred to bring the ID out to the correct tapping diameter, so the tap isn't overworked. I've also done it with a half round file, using the old threads as a depth guide. File until only the last vestiges appear as a thin continuous helix. If your eyes are bad, the ID can be colored before starting for visual contrast.

You'll also have to file away any excess chainstay, seat or down tube that extends into the shell because the taps need to go pretty deep and can't be bumping into stuff.

Be sure to use plenty of quality cutting oil, (preferably an oil with sulfur and chlorine, rather than a water base), and monitor your torque so you don't somehow jam the tap because of chip overloading or other issue.

Total time for the job is 15-20 minutes if you have the reamer, or about an hour if using a file.