The best option is to heat the iron until orange (temperature of decalescence 475 F to 525 F), either in a forge or with a torch, it will oxidise and leave behind Fe3O4.
If you live near an ocean, leaving iron in seawater for an extended period should create a coating of Fe3O4. Or, you might scavenge for some Fe3O4 coated iron near the shorelines - I found some last time I was near the ocean.
Since I am not near an ocean at the moment, don't know any blacksmiths, and wanted to speed up the process, I used the following recipe:
- First you will need to create Fe2O3 (red rust). If your iron already has a good coating of red rust, you may skip this part. If you need to make your own red rust, it is a little trickier since you will need to handle a few chemicals, but not bad. My recipe is not optimal, if you have Nitric acid and washing soda you can do better, but neither of these chemicals can be easily found at a hardware store - so, for most people, this recipe should be easier to implement. Near the concrete section of the hardware store, you will likely find something labeled as 'Muriatic Acid' - it is actually 30% HCL. Use your rubber gloves when handling any chemicals. Dip your iron nail in a mason jar of this acid and you will be left with a clean piece of iron ready to be oxidized. Now, pour another jar full of household Clorox bleach (or generic) and dip the cleaned iron into this solution. Allow the iron to dry in a glass dish - and you should have a nice coat of Fe2O3 (red rust).
- Now that we have a good coat of Fe2O3, we will convert this to Fe3O4. The Fe3O4 will appear as a black coating over the iron, where the red rust was formerly. Heat a pot of water on the stove until it is boiling and place the red rust coated piece of iron you created in the boiling water, after a few minutes it should turn black, now you have Fe3O4.