This is a tough one and can be answered in many different ways. If you want to be a plus handicap I think you need to be a very above-average ball striker. A lot of the skills necessary are going to come with your tee shots (keeping them in play with good distance) and approach shots (above 50% GIR). That’s not to say wedge play and putting aren’t important, but I believe ball striking is a pre-requisite to get to that level.
In my case, I think the thing that was (mostly) holding me back were poor tee shots. I’ve always been a good iron player, so it wasn’t a matter of hitting more greens when I had a “clear” shot at them. My driver was very erratic, and I would find ways to avoid it. So there were two major problems - putting myself in recovery situations quite often and taking a shorter club off the tee, which I’ve since realized was also costing me strokes.
The one skill I’ve improved the most is controlling the face of the club at impact. Since I have a very in-to-out swing path, that was crucial. If I don’t get this right, I’m either hitting a big block or a massive hook. I wish I had a simple answer on how to do that for everyone, and please realize that your issues might be different, which is why @CoryO suggesting that using a stat tracking tool like Arccos or Shot Scope is beneficial.
This is a massively simple explanation of what’s occurred over the past 5-6 years. I’d say I got better at every part of the game (including wedge play and putting). In a way, all of the articles I’ve written on the site are small pieces of the puzzle I figured out, but that are still relevant to all levels no matter what scoring goals you’re looking to achieve.
I realize we have a few golfers on here who are scratch level and a lot of others who are at other levels wondering how to get there. Please do not think that scratch golf is the level everyone needs to achieve. This has been a lifelong pursuit for me, and I’ve spent thousands of hours. Also, I know plenty of golfers who are 15 hcps who are much happier golfers than scratch players. Your score and hcap level are just one piece of the puzzle!