View Z shows a package of lobster clasps. I used one of these to close my necklace, but there are a whole bunch of different types of clasps that would work equally well.
If you leave long tails of floss at the beginning and end of your work, you might be able to use them to attach your closure. Thread one of the ends through a tapestry needle, and stitch the floss through the hardware to attach it. Views aa, bb, and cc show how mine looks while the stitching is in progress. Yours might look different, and that is perfectly fine.
View dd shows the clasp after it has been attached. After I stitched the clasp on, I continued weaving in the rest of the loose end as usual.
Alternatively, you might just want to weave in your loose ends and attach your clasp separately, and that would work fine too.
You'll also want to attach your pendant. I attached mine by stitching it to the center point of the necklace using a bit of the same metallic embroidery floss. This might or might not work for you, depending on how your pendant is constructed. If your pendant has a large ring, it might fit over the necklace easily. Most pendants will have some sort of mechanism allowing you to attach them -- if not a ring, there will hopefully be some sort of clasp or attachment. If not, you can always add any additional hardware necessary to attach the pendant to the necklace.