There are many beautiful and decorative clasps to choose from here at Golden Age Beads and most have the advantage of being easy to attach. The lobster clasp as shown below is a popular choice and it works well in various jewelry projects.
Clasps sometimes come with rings attached, but if not, you can use jump rings to attach them. Take a look at the following steps which demonstrate how to quickly add a clasp to chain. You can do this with a string of beads as long as you have a secure loop at the end to fix the clasp onto.
Open up a jump ring with pliers, slide onto the clasp and also the loop at one end of your chain or other material. When both are attached to the jump ring, close it up.
Use another jump ring to attach to the other end of your chain.
You can then attach your clasp to the jump ring at the opposite end to close it up.