Technical Steps for a Book Cover
Simple Version

If you have high quality photos you've taken yourself, certainly use those. But I caveat they have to be high quality, crisp, professionally done photos. If your photo is iffy it is going to reflect on the entire book. People do judge the book by its cover.
Go into Paint, PhotoShop, or whatever program you use and set up an initial image size of 2318 wide by 4500 high. This is the size of the entire cover image.
If you want a white background, like in my example, make sure you give it a black border around the edges. You want to make sure the book image is separated from the white background of the screen.
Use the text tool to add in a title, subtitle, and author name. In general you want these to fit the space left-to-right with some margin. Don't jam your letters up against any edge of the book. You always want space on all sides.
Open your chosen image and select it. Copy it. Now go back to your book cover and paste in the image. You should be able to use the arrow keys to slide it around to where you'd like it.
There are a wealth of YouTube videos to help you learn the basics of most paint programs. Spend even an hour learning the basics like how to resize images and how to slide them around. These skills will come in handy for ALL sorts of reasons going forward.
If you're truly stuck, read my advice on working with a paid helper! But, again, it's well worth it to consider this a skill worth learning and to keep practicing. The better you get with graphics, the better you can do in all sorts of aspects of marketing and promoting your book.
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