The fastest way to do it is to memorize the four note "core" of BCEF and the two notes above them, D and G, then memorize the two notes below them, A and D. Play in the key of C for about a month straight, and don't falter from it, stay in the key of C only until you can visualize it up and down the fretboard, then start on this task of BCEF, then the two notes above and below that.
Divide the fretboard into two-string playing areas, the EA area, the DG area, and the BE area, and then connect all of those four-note per string patterns together (all of the ABCD-DEFG patterns). Once you have done that, then memorize the remaining pattern of EFG-ABC, which is an offset pattern. Then practice connecting every one of these patterns while naming the notes out loud every day for 10 minutes every day for a week, and go back to it at least once a week every week after until you know every one.
Once you know all of the natural notes you will know all of the enharmonic names for all of the notes in between with little more than a second's thought.
