Have you a metal ring sizer? Or something metal that's the right size for the ring you want to make? If so then just wrap your strip of clay around that and cure it right on the metal. Don't use wood or you'll never get the ring off once it's cured. You can decorate the ring before you cure. It's easy when the band is already on the sizer. Then stick the base of the sizer into a mug or something to keep the ring from touching anything in your oven and POOF! all done. :) Don't forget to take photos for us! :D
If you can find something to put the ring on before you cure it, you can use a wand or knitting needle or just your finger to smooth over the join then put one of your decorations over that spot. You could also use a piece of flashing as armature but that's a lot of work for only wearing it a couple days. I say just go for the clay and avoid using it to knock on solid oak doors. :D
Are these for an event or something? (You mention wearing them for "a couple of days".) Because I wouldn't trust polymer clay to hold up very well for longer than that.
Also, definitely I would use well conditioned strong clay (Kato, Premo, Fimo) and cure for at least a half hour at the hottest setting safe for the clay. You want the flexibility of properly cured clay, not the brittleness you get if it's not right (or that you get from Sculpey.)