Rust bluing a lock (or any part) is extremely easy. Like most things, the prep is the most important and challenging part.
It is basically just browning but with a boil after each rusting session. You can use your favorite browning solution as well. As with browning, more dilute solutions and/or shorter rust periods will lead to finer, smoother finishes. Boiling is generally with "pure water" (deionized, or distilled, or reverse osmosis) to avoid mineralizing spots when the hot part comes out of the water.
If you want more instructions - I could send you some that Jack Belk wrote years ago. They are pretty simple. Jack was a renouned gunmaker.