There are many posts here with great detail and pics if you google "oxalic acid site:bikeforums.net" - but in brief:

1. Strip the bike to the bare frame
2. Clean off any oil/grease
3. Get a kiddie pool
4. Submerge the frame in a dilute solution of Oxalic acid and water (about 1 level tbs/gal)
5. Check periodically
6. Remove any residue from frame by flushing with fresh water and soft bristle brush
7. Touch-up as needed.
8. Apply Frame Saver or Boeshield T-9 to all interior surfaces of the frame.
9. Treat frame with Scratch-X as needed, and apply a liberal coat of good quality wax.

PS - if there is heavy scale on the frame it's best to remove the scale mechanically first - then follow up with the OA bath. You will have to seal any exposed raw metal afterward of course with touch-up or clear.