The basic components for this type of ring are a vintage watch case with back and a suitable decoration, plus your favorite ring shank. I chose a cigar band in silver plate from http://www.bsueboutiques.com/item/Cigar-Band-Style-Ring-5533 and a resin floral bar from there as well. They also carry watch cases from time to time in this style.
Next rivet the case to the shank. I like to use this technique for getting good position on the holes. Punch the first one then rivet it then punch through the ring into the second ensuring an accurate fit.
Next fit the flower decoration into the base and sand or file the top edges to fit the watch case top. As you can see there is a cutout for the stem of the watch to protrude. If you turn the top around so its notch is on the opposite side you wont have a hole into the case. Of course you could fit a crown to a stem and drill through the cab inside and glue it in place for added decoration.