How do I make custom backgrounds?

Hi there! I’m a cover and splash artist who loves to make art for people and I really what to try to make custom backgrounds! Can anyone explain to me or make a tutorial. It would be greatly appreciated!


A lot of people use picmonkey!
But I don’t and I kinda keep the app I use a secret.
But they look like this:

♛Interior Backgrounds♛

♛Exterior Backgrounds♛