The Open G Chord
Some Theory
The (open) G chord is made up using the 1st, 3rd and 5th notes of the G major scale which are G - 1st, B - 3rd and D - 5th. When playing the open G chord we get a G note on the 3rd fret of the low-E string, a B note on the 2nd fret of the A string, a D note on the open D string, another G note on the open G string, another B note note on the open B string and another G note on the 3rd fret on the hi-E string.
Playing the Open G Chord
I've seen some people play the G chord using fingers 2, 3 and 4 but again this doesn't make much sense to me and the more traditional way as shown in the chord diagram is a better way to learn it in my opinion. That said, there is another way we can play the G chord (as shown in the video) where we introduce the 4th finger on fret 3 of the hi-E string (which replaces the 3rd finger), and then place the 3rd finger on fret 3 of the B string. I am going to recommend that you learn to play the G chord this way as 1. There are loads of songs that use this type of G. 2. Using this type of G also gives us a 3rd finger anchor for when changing to the D chord (which tends to happen quite a lot in songs). 3. It introduces the 4th finger - lots of people tend to ignore the fourth finger as it's the weakest finger so it's good to get using it quite early on.
Technique
To apply the correct technique for playing open chords, I've devised six small instructions (as shown below) to help you along. Note that these 6 instructions are the same for nearly all of the open chords.
1 Correct thumb position
Make sure that the top of your thumb is visible over the top of the guitar fretboard around about behind fret 2. If you're able to bend it over the fretboard so that it can reach and gently touch the low E string, then you have it in the correct place. If it's not placed high enough and you can't reach the low E string, make sure you raise it up a bit so that you can. It's recommended in some beginner books that that the thumb be placed much lower on the back of the guitar neck, but for me this doesn't create a solid technique for playing open chords in contemporary playing.
2 Correct wrist posture
For everything that you play on the guitar, you want your body to take up the most natural posture possible - this includes your wrist. If you have your thumb in the correct position, you'll notice that your wrist takes up a comfortable, naturally straight looking posture. If your thumb is too low on the fretboard, you'll start to see your wrist bend unnecessarily. Just make sure that your thumb is right, and your wrist will also be right.
3 Fingers curled correctly
When playing open chords, make sure that your fingers are curled at each knuckle. We don't want any "flat fingers" on the guitar (not yet anyway). Each knuckle will be curled slightly differently for each chord so make sure you're watching the video carefully and experiment with what works best for you. If your knuckles are curled appropriately you'll find the next rule " pressing down with the finger tips" quite straight forward.
4 Press down with the finger tips
If you've followed the first three rules correctly, you should now be able to press each note down using the Finger Tips. For open chords, we always want to be pressing down with the tips of our fingers, even though sometimes (depending on the chord we're playing) certain fingers will seem like they're pressing down with less of the tip than other fingers. The most important thing is that all the notes are ringing out, so again, experiment with what works best for you. Also, check the video carefully for these subtle differences.
5 Finger angles
Now at this stage when your fingers are pressing the notes down, your fingers should automatically be angled in a diagonal direction and pointing up towards your face (not directly upwards) - and this is what we want. Each finger wont be angled exactly the same but this is fine. Your fingers should do this quite naturally, and again, this conforms to keeping the hand in the most natural state possible.
6 Press the notes down right up close behind the metal fret wire
And finally, whenever we press a note down on the guitar we want to be pressing down right behind the metal fretwire in the fret we're playing in. It's no different for chords either. When playing the G chord, it's not too tricky to place each finger right up close behind the metal fretwire, but finger 3 has to give way to finger 4 in fret 3 so just ensure you place finger 3 as close as comfortably possible to the metal fretwire in fret 3.
Tricky Spots
As for tricky spots when playing the open G chord, just be sure not to have the 1st and 2nd fingers too flat on the low-E and A strings, as sometimes the underside of these fingers can cause the strings directly below to sound muted. Also, make sure that you apply enough pressure with the 4th finger on fret 3 of the hi-E string as this finger tends to be a little weaker than the others when you first start using it.