I'll begin with a very simple cover which is also a cover of this tutorial book.

Here, I've used three resources.
Sky is by BEAPANDA on deviantart.
Sea by sweetpoisonresources
Red Haired Girl by Barques

Apps Used : ibisPaintx
Polarr

Insert sky 👆

Insert sea 👆

Insert png 👆

Insert title 👆

Done here.
Now let's move to Polarr for filters. You can use any filter you like. I'm using one called 'beyond horizon'.

And it's done
A simple cover.

Tips
(for title)

Try different fonts 👆

If there are more words, try using different fonts. A serif and a decorative, groovy one. 👆

You can go with a mixture of uppercase and lowercase.

Again, try different fonts 👆

Try different locations 👆

Try adding shadow 👆

Or glow 👆

Remember, match the font style with the mood 👎

Placement matters, especially when you have space where you can insert your text 👎

Yep, placement really matters 👎

You can go wild but it doesn't always work.

Do more experiments with typography

What kind of balance do you want to establish?
Typography > Background
Or
Typography < Background
Or
Typography = Background

All three work 👍