In this GIMP 2.10.10 tutorial, I show you 3 methods for adding a border to your images. The first method is the fastest/easiest and involved the Script-Fu “Add Border” feature. The second is also easy and involves a layer mask. The third requires more steps but is the recommended method, especially for printing your images with a border.
This GIMP basics tutorial is a great tutorial for beginners and should be easy to follow.