The next step is to increase the canvas size to whatever dimension you would like the frame to be. The proportions and size of the frame are entirely a matter of personal preference and style: this is something to experiment with for yourself and see what you prefer. For the purposes of the tutorial, I will keep things at the standard 4:6 ratio.
GIMP has some useful controls in the canvas module for this, including the “chain” that will keep the canvas true to its original proportions or, if deactivated, allow different proportions. There is also a function that allows you to center the current image within the new canvas, or manually type in where you would like it placed relative to any side.
After you've found a canvas size that satisfies you, go ahead and create a new layer. If you're going to be using multiple layers, it's a good idea to label it.