The simplest is just sewing a strip onto the back at two ends, making a loop for the belt. If you’ve a bigger piece of leather, you can make a wing on each side with a wide hole that the belt can go through. Or the other direction, up, then loop back down and sew or rivet to the bottom.

No belt loop on this one, I just wanted a cover for it to better keep it clean while its knocking about in my pocket or backpack. That said its a good starter project