See the instructables link above if more context is needed. These are the basic steps I took after tweaking that guide for my setup.

1. Grind or cut soldering iron tip to desired shape and size
   1. be careful if it has been used for lead soldering when grinding, wear a mask and vacuum any dust
   2. sand with 500 grit or so to get a reasonably smooth finish
2. Pour vinegar in small cup, 300mL or so is plenty, doesn't need a lot
3. Add in pinch of pure salt and stir
4. Cut nickel strip long enough that it can sit in the glass jar and reach the bottom
5. Attach positive and negative to the nickel strips
6. Apply 6V or so, limit current to 0.1A or so (?)
7. Let run for a few hours, the vinegar should turn greenish blue color
8. Remove the nickel strip that has dissolved into the solution (mine had holes in it at this point)
9. Clean soldering tip to be plated
10. Optional (electroclean, unsure if needed or useful): 
    1. Attach soldering tip to positive lead and old nickel strip to negative, run at 3V for 60s or so to dissolve
    2. Can be done in another cup to avoid contaminating solution 
11. Attach soldering tip to negative lead of the power supply, nickel strip positive lead
12. Hold the soldering tip so that only the portion that you want to wet is submerged in the vinegar. But all raw copper must be plated.
13. Set power supply to 5V and current limit of 5mA
14. Run for an hour or so
15. Set current limit to 10mA
16. Run for a 2-6 hours
17. Rinse off vinegar, tip is ready to use.

In the beginning you may want to check the status of the plating every 15 or 30min. If you see black soot, this is some kind of contamination (not sure if my nickel strip was not pure or it was from something else, do not allow any alligator clip or any other metal to be submerged in the vinegar). If you see its not plating evenly, you can remove and clean the areas that are not plating, carefully scrape or sand, then wipe and re-submerge. Maybe with better cleaning methods the plating would be more even, will keep trying.

In the beginning voltage in a small cup with items ~2cm apart was ~2V then it slowly increased with time. You can try higher voltages and currents, but these low numbers should improve the quality.