There are a number of short tutorials on YouTube for this. This one
https://www.youtube.com/watch?v=jC3jpjgesXA
or pehaps this one
https://www.youtube.com/watch?v=icNXrgD08qE
may assist, although there are plenty there. It will also depend on which photo editor that you're using - you may have to search a little to find the exact one you've got.