1 - Notes on one string

First thing to do is to try out the musical alphabet on one string. If you take sixth string (E) you should go:

Try playing fret after fret while spelling it's note, and also try it backwards.

2 - Open strings – 12th fret

I'm sure that all of you know open strings (1st string E, 2nd string B, 3rd string G, 4th string D, 5th string A and 6th string E). The same notes are on 12th fret, which are octaves.

3 - Basic chords bass notes

Other thing I'm sure you all know, are basic open and barre chords. See where their bass notes are and memorize them. Also try to make them sharp and flat. Put them as your mental keypoints. Compare other notes around them to those notes. Focus only on D, A and low E strings up to 5th fret.

4 - Tune notes

Most people usually tune their guitar the following way (if not with a tuner). 5th fret on second string has the same note as first open string, 4th fret on third string has the same note as second open string, 5th fret on fourth string has the same note as third open string, 5th fret on fifth string has the same note as fourth open string, and 5th fret on sixth string has the same note as fifth open string. Add those notes to your keypoints.

5 - Same notes on adjacent strings

Learn how much frets are between same notes on adjacent strings. Add this information to your note keypoint referance. Here is the example:

Other thing you could do is practice the principle through whole neck, in both directions.

6 - Octaves

Besides helping you learn the notes, playing octaves can sound great in solos and melodies because they empower the melody line with higher or lower note. Try to play both F's on 4th and 6th string, and both F's on second and fourth string. Then try to find what other options you have with this on your neck, for all other notes, too.

Another octaves you have are the octaves on adjacent strings:

Experiment where are octaves for all other open string notes, and add them to your keypoints. The main reason for having keypoints is that you don't have to go all the way from open string to get to the note you currently don't know.

For example, if you don't know which note is on 8th fret on fourth string, you don't wanna go from open fourth string, note by note, to get to 8th fre. It is much simpler to learn the keypoint of 7th fret, fourth string being an octave to open fifth string (which is A note), and then you easily see that 8th fret on fourth string holds A# note.

So learn as much keypoints as possible, and rest of the notes will come easily.

7 - Above 12th fret

You can use all the principles mentioned before. Another thing is, after you know the note keypoints, to get them on frets above 12th fret, just add 2 frets. For example, you know that E note on fifth string is on 7th fret. To get it above 12th fret, just add 2 frets on last digit, so it will be on 19th fret. Work on this only when you actually need those notes, because learning without purpose won't benefit you much.

The other thing you can do is trying to visualize the whole fret without actually having guitar in your hands. Or randomly choose some fret on guitar neck, and then determine it's note. You can do this anytime, and it will help you a lot.