Superimpose:

Step 1: Open up your image with the constraints 2:3. Then open up a gradient. I'm gonna include some at the end of this chapter!

Step 2: Put your gradient image on screen. Hit the crossed arrows to fit it to your picture. You can turn some of the opacity down to make it less bright. That's what I do.

Here's my finished product:

Picsart:
Step 1: open up your image. Make sure the constraints are 2:3.

Step 2: Put your gradient on screen and turn the opacity down.

This is the finished product:

Gradients: