First of all, remember that real leather is a natural material and it will stretch a bit on its own over time. But if you can’t wait for that, there are a couple of ways to stretch leather straps on your own. A note of caution: if you’re trying to stretch the straps on a nice leather bag, please don’t do it yourself. Bring your bag to a professional so you don’t ruin it. If that’s not an option, and you’re a little bit crazy, you can try one of these methods:
Water Fill a basin with warm water, submerge the leather straps in the water, and gently twist or knead the straps to help them absorb the water. Leave them in the water for up to ten minutes; then, gently stretch the leather straps to the desired length. If the strap or straps can’t be removed from the bag (to stretch in a straight line), you can fill the bag with books to weight it; then hang the bag up and let the weight of the bag do the stretching. After the leather has dried, treat it with a leather conditioner.
Fill a spray bottle with equal parts rubbing alcohol and water. Generously spray the solution onto the leather straps, and proceed with stretching in the same manner described above. After the leather has stretched and dried, treat it with a leather conditioner. Again, please use your head: just because it’s possible (and not even very difficult) to stretch leather at home doesn’t mean you should. Fine leather is an investment; treat it that way.