What makes someone's personality attractive (there are limits to what you can do about appearances) is self confidence and giving the impression that you have something interesting going on. Essentially you need to seem exciting to be around. That doesn't even mean like skydiving but just having hobbies you are passionate about.
Self-improvement might be one way of getting there but it's mainly a case of being comfortable in your own skin. I think this is why so many incels get confused about going to the gym not immediately making them desirable.
Lastly, you need to be genuinely interested in any potential partner as a person. People can sense when they are talking to someone with ulterior motives. This might be the biggest hurdle for hetro mysogynists who don't see women as people.