So let's say you have a favorite pair of shoes. Like, hands down, the shoes you wear every day because they are super comfortable to walk in, and you can ride your bike in them, and they look pretty good too. And then, let's say, one day one of the buckles on a shoe strap just snaps in half. Out of nowhere, just breaks into pieces.
You probably say to yourself, "Well, I'm sure a cobbler could fix this," and you go to your local shoe repair place. But then, let's say, the cobbler shows you buckles that are both the wrong size and color (the only ones he has!) and also expects $30 for the privilege of uglifying your favorite shoes. Do you give up? No, of course you don't give up. Instead, in what you think is a brilliant move, you call the shoe manufacturer... but to no avail, because they don't stock replacement hardware. This is probably the moment that you decide to take matters into your own hands.
After a careful Etsy search, you find someone who makes almost identical buckles to the ones on your shoes. "Hooray for independent small business craftspeople!" you probably exclaim to yourself, as you ready your tools: the new buckle, some waxed thread, a seam ripper, and a curved needle.
Your first job is to pick apart just the bit of the seam that holds closed the loop of leather that would usually attach to the center of the buckle. In this picture, it's the darker leather piece (you can see the hole that the buckle's prong goes through).
Once that's done, you thread the new buckle onto the now open loop, making sure to put it facing the right way.
Now it's time for the finicky part - stitching the loop closed again. The advice for leather is just to reuse the old stitch holes. After tying a knot at the end of your waxed thread, you start a little bit before the first seam-ripped stitch, going over old still-in-place stitches to secure them. You try your best to sandwich the knot between the layers of leather.
Slowly, slowly, you keep going from hole to hole and then double back over again. If your thread happens not to match perfectly because apparently your laptop's screen colors are not true to life, and you bought your thread online - well, it's no big deal because the new stitches will be hidden once the buckle is buckled anyway.
After the last stitch is in place, you tie a small knot on the side and then slip your curved needle through the layers of leather to the other side. This will both pull your knot in between the layers of leather so it's not poking your toes, and will make it more stable.
And, just like that, you have defeated the forces of evil shoe destruction and have emerged triumphant with your favorite shoes intact! Winner, winner - chicken dinner.
(The fixed shoe is on the left.)