Attaching clasps can seem complicated when you have never done it before, here are some tips and a little demonstration:
Once you have slipped the fabric into the clasp (it is hollow), center the clasp and start sewing by stitching in every second hole to the end, then back to the center, continue to the other end and return to finish where you started.
If you have a thick velvet-like fabric, don't hesitate to use a pair of pliers.
Last piece of advice: use a thread that matches the lining, as it is the inside that it is likely to show. Now you can go for it!