Before you sew on your buckle, make sure it fits into the slot you created earlier. Assuming it does, you can begin to stitch it in.

You'll need to draw out a length of thread, cord, sinew, floss or other material you intend to sew this project together with. If the thread you've chosen is like mine, and I mean unwaxed linen thread, you'll need some beeswax to run it through before beginning to sew, to both lubricate it as well as give it some additional strength and water resistance. As a general rule of thumb, you'll want it to be roughly long enough to cover all the stitching at least three times so you can properly tie it off in the end.

For this portion you'll need only one needle and to keep your awl close at hand. Once you've threaded the needle you'll want to pass it through the first hole and leave a small tail which you'll use to tie off on later. I prefer to start near the edge of my piece and usually right next to the buckle as pictured above. Next I like to pass the needle around the outside edge of the strap and back through the hole again.

Doing this will prevent the edges of the strap from rolling up and putting stress on the thread causing it to eventually fail. This same process is repeated along most of the holes near the edges, including the bottom of the strap itself to give my project a little more longevity. As far as the rest of the stitching goes, it's as simple as passing the needle through the holes and working your way back over to the starting point to tie off the left over thread and it should go without saying that you need to pull tight to make sure everything is joined properly.

I like to try for a nice pattern, so I typically will work down half the holes first, and then attempt to duplicate the stitches on the other side, which again, you can see pictured above. One you are able to work back to where you began you'll need to tie off with a square knot and pull the portion with the needle attached through your first hole. The idea here is to sandwich the knot inside the leather so that it's less likely to come untied.

Once you have the buckles attached you can attach them to the main body of the bag. Seeing as we have already punched the holes at the bottom of the buckles, but not in the bag we need to plan out where they'll go. I decided to place one strap and buckle every three inches down my bag right around the edge of where the top flap was going to end. After measuring out the locations with a ruler and a square, I used the holes punched in the end of the buckles to mark where the new ones should be in the bag.

I also added extra holes around the perimeter to over stitch the edges of the strap, similar to the way we earlier stitched around the edges near the buckle. Once the holes are punched, and assuming you are pleased with the results, you can affix the buckles to the bag's main body, using the same pattern you developed on the buckles, or a new one if you like. Regardless it is essentially the same as sewing the buckles into their place.