I used those on a purse that I made ... works great!! I loved them.
Yes, you want to sew them into place inside a fabric pocket or tab. Keep the stitching as tight as you can to the magnet, as within time the plastic will open up and the magnet will escape. And make sure you get the magnets facing the right way, or else they aren't going to work!
The challenge of stitching ... all the metal parts in your machine and the magnets. It kept grabbing on as I worked at it. A bit annoying, but take your time and you'll be pleased!