Just rub it in, the first couple of coats will get absorbed quickly, then just keep rubbing it in until it won't take anymore or until it's dark, then buff it with a clean rag.
If you're gonna do it do it properly, clean the fretboard first, and get the grime from the side of your frets out with a toothpick.
You could also masking tape your fretboard and wire wool the frets to a high shine before you do this for the best result.
