Typical soldering iron tips are copper core with an iron plating where the solder is expected to stick, and chrome where its not. On "high performance" tips the iron plating is often quite thin and will quickly wear out, iron does not conduct heat as well as copper.
Plating iron is not very easy, but nickel can also be plated and will wet solder well. Longevity will likely not be as long, but I don't know what the difference is.
See the instructables link above if more context is needed. These are the basic steps I took after tweaking that guide for my setup.
Grind or cut soldering iron tip to desired shape and size
be careful if it has been used for lead soldering when grinding, wear a mask and vacuum any dust
sand with 500 grit or so to get a reasonably smooth finish
Pour vinegar in small cup, 300mL or so is plenty, doesn't need a lot
Add in pinch of pure salt and stir
Cut nickel strip long enough that it can sit in the glass jar and reach the bottom
Attach positive and negative to the nickel strips
Apply 6V or so, limit current to 0.1A or so (?)
Let run for a few hours, the vinegar should turn greenish blue color
Remove the nickel strip that has dissolved into the solution (mine had holes in it at this point)
Clean soldering tip to be plated
Optional (electroclean, unsure if needed or useful):
Attach soldering tip to positive lead and old nickel strip to negative, run at 3V for 60s or so to dissolve
Can be done in another cup to avoid contaminating solution
Attach soldering tip to negative lead of the power supply, nickel strip positive lead
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.
Set power supply to 5V and current limit of 5mA
Run for an hour or so
Set current limit to 10mA
Run for a 2-6 hours
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.