We concentrate on the notes on the first 12 frets since everything just gets repeated over the next 12 frets:
0. Primer: The notes at the nut (and also on the 12th fret) from low- to high-strings are: E A D G B E. ("Eddie Ate Dynamite; GoodBye Eddie!")
1. Learn the notes on the 6th string. Once you do so, you also automatically know the notes on the 1st string (the notes are exactly the same!). Here are the notes on the 6th and 1st strings:
2. To learn the notes on the 5th string, just remember the A Minor shape (I show a 3-Note-Per-String shape here) from the 5th fret of the 6th string. Since A Minor is just C Major; there are no sharps or flats to worry about. That shape delivers: A, B, C, on the 6th string, and, crucially, D, E, F on the 5th string:
The note that is 1 whole step up from the F and 1 whole step down from the A on the 5th string of the 12th fret is obviously G. The note that is a whole step down from the D is obviously C. From there, B is obvious (half-step down from C). A is the open string.
3. The notes on the 4th string are the same as the ones on the 6th string except up a whole step. So, just visually "skip-a-string-skip-a-fret" from the 6th to the 4th strings and you get those notes (an example from A Minor, but applies to ALL notes from the 6th string, of course!):
4.For the 3rd string, it is easier to look at the notes on the 1st string (see STEP 0 above). From 3rd to 1st it’s just "skip-ONE-string-skip-TWO-frets:
5. Finally, for the 2nd string, consider the A Minor CAGED shape but now working backwards from the 1st string. The notes on the 1st string are obviously A, B, C. On the 2nd string, they are E, F, G (going back from A):
Hence, on the 2nd string, after the G is an A that then goes to the B on the 12th fret. The E goes down to a D then to a C on the 1st fret and then to the open B.
6. As mentioned at the beginning, everything gets repeated after the 12th fret: