Leather crimps are specifically designed to allow you to attach a clasp to a piece of leather. You may find that different brands look slightly different, but essentially this kind of crimp has a circular end onto which you can attach a clasp and two pieces of metal that can be folded over to encase the leather.
To use a leather crimp, carefully place the end of your leather string against the back of the straight piece of metal. Use flat pliers (or in some cases the metal is soft enough to use your fingers which I find gives a little more control) to fold first one side of the sticking up metal over your leather. Then fold the other side over on top. Finally, use the flat nose pliers to squash the crimp as flat as possible so that the leather is held securely.
It is possible to close leather crimps using round nose pliers, but these tend to make an uneven indentation in the metal on the crimp, so it can be more difficult to get a neat finish. If, however, you are new to beadwork and only have a limited toolkit, then you can use these at a pinch. It’s probably not worth investing in flat nose pliers if the only thing you are ever going to do is close the occasional leather crimp! If you are using round nose pliers, then just make sure you re-position the pliers to flatten any areas of the metal that are slightly raised – rather than flattening the entire area of metal in one go, you can flatten it gradually, small area by small area.
I find it is best to cut the leather to the correct length and line the end of it as close as possible to the bottom of the circular hole in the crimp. It is best if a tiny bit of leather protrudes from the end of the crimp (see the photo) as this helps the crimp to grip, but make sure that this is only tiny – you don’t want a long length flopping around as is looks unattractive and also gets in the way of the hole through which you will attach other findings.
Finish your jewellery by attaching a clasp to the hole at the end of the leather crimp. If your clasp has a natural opening on its ring, then you will be able to thread this directly onto the end of the crimp. If the ring at the end of your clasp is closed (see closed rings), then you will need to use a jump ring to attach the components.