Book cover designers are awesome.
That is, *IF* you can afford them.
For writers with low or no budgets, you can attempt to do things on your own. These steps can help.
1. Look at Best Sellers
For example, if you're writing cozy mysteries set in London, pull up the most popular books in that category this month.
2. Note Patterns
You'll notice that there are often-used elements present in those top 10 or 20 books. They may use similar color schemes (black-teal, gray-white-yellow, orange-purple-green, etc.), graphics (someone running in a forest, a man with his shirt off, a pair of shoes on a sidewalk, etc.), or text (typeface, font, colors, character spacing, etc.).
Write these patterns down so you can reference them while designing your own cover.
3. Start Designing
Now that you know what the pros are doing, choose which of these common elements you like and start incorporating them into a custom design.
There's a lot that goes into creating a strong cover that serves you well as your book's first impression to potential buyers. But these steps can help you get oriented to the process of trying to handle cover design on your own, if you must.
Remember: There's no such thing as a perfect anything in life. Including covers. There will be people who love your cover and people who don't, no matter how much research you do, or even whether or not you hire a professional. One great thing about being an indie author who manages their own literary career is that you can make changes in the future. If five years from now your cover looks outdated, you can update it.