Darwin Deez

Here’s what I did: Take the E strings off your guitar, both. Take the B string off, too. Tune the A string down to F# and the G string up to G#. Practice figuring out the chords to songs you like using one chord shape and moving it up and down the neck. Your goal is ear training. Can you recognize whether a chord is major or minor? (Test yourself here if you’re not sure.) I bet you can do that. What you want to teach yourself is the sound of a ii chord vs. a vi chord, or a IV chord vs. a I chord.
Here’s a great place to start: Ever heard “In My Life” by the Beatles? The most beautiful part of the song–the part where he sings the title–features a very distinct and recognizable chord: the iv chord (“minor four chord”). This was the first chord I was able to recognize in other songs. Can you hear the iv chord in The Aisler’s Set – Chicago New York?
IV is the roman numeral for 4. In this notation, capital roman numerals indicate a major chord and lowercase, minor.
The iv chord is a bit of a rule bender as it turns out. Most songs use only the following chords: I ii iii IV V vi. So the IV chord is usually major. John Lennon broke the rules and made it minor. Genius. You too can break the rules once you learn them.
I ii iii IV V vi
Each chord is built on the corresponding note of the major scale. You know the major scale. So when you are guessing which chord is happening in songs you like, use this list. The guitar tuning will force you to see the distance between the root notes of each chord in the list over and over. Right there on the guitar neck. ii is 2 frets up from I and 2 frets down from iii. vi is 2 frets up from V and 3 frets down from I. Et cetera. You will begin to learn this by heart because of how similar all pop songs are. Even in minor key songs, your reference list is the same. (The minor scale is the same as the major scale, it just starts on the 6th note instead of the 1st.)
The sound of the I chord is important but easy. It’s often the first chord in a song and the easiest one to find. It’s the chord which feels like home. From there, use the list (I ii iii IV V vi) when guessing at the other chords in the song.
The benefit of this system is that you do not need to learn any letters (A, C, G#, etc.) only roman numerals, which represent the harmonic function of a chord–its essence, its true sound–which is what you’re after. Once you have trained your ear to recognize that, you’ll be able to play any song on guitar. On your regular guitar. Which is otherwise too confusing.
Just to be clear, a the notes that make up a ii chord are not any different than the notes that make up a iii or vi or iv chord. All are minor chords composed of a root, fifth and a minor third. The difference is that, within a given chord progression, the ii chord does have a different sound than the vi, and it’s learning to recognize that sound that will help you play any song on guitar.
When you figure out one chord in a song (and identify it as the I chord of that song’s chord progression), you can then start to make educated guesses about what the other chords in the song are. You can quiz yourself. This is literally how I learned it and I use it to write songs all day. Have fun!