Learning to design well is like learning to code well: You do it a lot until you improve. Examine "good" designs, thoroughly. Keep notes of sites or apps you find attractive, and try to find out why. Emulate those things.
Also, read some actual design theory books. Theory is to design what algorithms are to code.
I did this and it worked for me. I taught myself design after finishing a code-heavy computer science degree. I learned how to use Photoshop quite quickly along with some design theory but it took time to get really good.
I used to look at those sites that were popular a few years ago that list well designed websites, and take inspiration from one or several of them at a time by combining the elements to create a new design.
It helped that I was working for a company that wanted a new design created to a brief each week as it helped me to get better quite fast.
Reading a lot of design, UI and UX blogs to understand why things worked was crucial to my development too.
Also, read some actual design theory books. Theory is to design what algorithms are to code.