How and where can you use these? Thanks for any help.

I know this post is a month old and maybe you found out already, but I figured I'd answer for anyone else looking here.

When you are looking at CW shops, there is a little blurb on the page telling you if you have a CW voucher. If you do, you can use it to purchase any CW (that's not password locked, I'm assuming anyway) that is 700 CSC or less. I'm so happy to have these vouchers because I can finally get some cute items without going broke irl haha.

I did but thank you anyway. :D