Most CD are write-once, read many times. There are some that can be erased and such, but you probably don't own any and you probably don't want to own any--the extra cost is rarely worth it when you can buy a pack of a hundred and throw them away.
CD have to be "finished" which is a just a way of putting enough info on them so that any computer can read them. Once they are finished, you can't put any more data on them--the finish info says what's on the CD, and if you put more, the info would be wrong, and you can't erase a normal CD. So normally you would put the data on the CD and finish it; if you do daily backups, you will run through a CD every day.
Therefore you have to start with a blank CD. Whether or not Word can see it depends on your computer and how it set your CD drive up. If you don't see it, the procedure would be: save the docs, copy them to the CD, finish the CD, the end. You cannot edit them from the finished CD but if your machine crashes, you can copy them from the CD onto the hard drive and edit there.
If Word doesn't see the CD, you can still sort of use it as a backup by copying files to the CD without finishing it, but it's kind of pointless because then only your CD drive will be able to read the unfinished CD (because only your CD drive knows what's on there before its finished) and if you computer crashes, you may have a problem reading the unfinished CD.