When learning anything on the guitar, I like to use pro players as an example of how to execute specific techniques, and it’s no different for open chords.
Closed Grip for Open Chords
In the case of playing open chords, if you watch the pros you’ll notice that they all play open chords with a Closed Grip, so this is what I usually advise.
To position your hand in a Closed Grip, gently bring your thumb over the top of the guitar neck so that it’s able to very gently touch the top of the low E string. From this position, the rest of the open chord technique kind of falls into place.
Press Down with Finger Tips
Now, whilst keeping your thumb in position, place your fingers on each note using the tips of the fingers. The exact part of the fingertip will vary depending on which finger you use and which chord you play.
Once the fingers are all pressing down with the fingertips (whilst the thumb is still over the top) you should notice that all the fingers are nicely curled. This is great!
Also, when fretting each chord, your fingers should naturally take on a diagonal shape of sorts, where the fingers point upwards towards your face as opposed to straight up.
The advice in this lesson will work wonderfully for most people. But if you’re someone with smaller hands or chunkier fingers, you might need to experiment with Open or In-Between Grips as opposed to a Closed Grip as recommended here.