If your leather strap is starting to curl, there are a few things you can do to straighten it out. First, try running the strap under hot water for a few seconds. This will help loosen up the leather and make it more pliable. Then, lay the strap flat on a hard surface and use a heavy book or other object to weigh it down. Leave the strap overnight to dry, and it should be straight in the morning.
If the above method doesn't work, or if your strap is severely curled, you may need to re-wet and reshape it. To do this, soak the strap in warm water for a few minutes, then remove it and gently stretch it out to the desired shape. Once you have the strap shaped the way you want it, dry it thoroughly and apply a leather conditioner to help keep it supple.