It's something I've wanted to achieve for years and years.
Sadly, I'm still not even close.
I find it amazing that I meet new players down the club, who've hit less golf balls in their whole life than I've hit in a month....and look! They have a straight (or near straight left arm)
I personally believe achieving it is a mixture of keeping your left arm close to your chest and getting the correct width/bend in your right arm at the top of the swing.
However, you can play good golf without a straight left arm. I know a few players and even a very good professional.
If there was a magic pill, I'd have taken it and probably overdosed too.
Stand a if your at set up. Lift arms straight up in front of you until horizontal.hinge wrists so that club is now facing the sky. Turn shoulders until left shoulder is under chin. That should give you the right feeling
My left arm refuses to stay straight and feels like Im ripping it out of my shoulder socket if I try too hard to keep it straight. Given up trying to keep it straight, doesnt seem to cause me many probblems, or Angel Cabrera for that matter
This is something I'm really stressing about at the moment and was even going to post a vid, unfortunately it's a habit I've practiced sinced I started playing golf a year back and didn't realise till a few weeks ago :-( I was getting really down hearted about it at the range earlier until I stepped back and thought hang on I'm stressing over this problem but still landing on or within 15 yards of a 150 yard green with 90% of my shots but I'm still not happy and want it sorted :-(
A slight crescent shape in the left arm in the backswing is not only OK it's a good position. This will prevent tension which is more of a problem.
I think the best way to consider it is to keep your hands as far away from your head as possible and keep the elbows close together without feeling undue tension.