Measure the stones and cut the backing material. The backing material can be 22, 24, or 26 gauge sheet metal. Which gauge is chosen depends upon the bead or cabochon chosen.
If the back of the bead is not flat and/or will not sit flush on the backing, or is larger than those used here, a heavier gauge may be preferable. Since earrings do not get a lot of heavy use, such as bracelets, and even pendants, a safe bet is to use 24 gauge sheet.
To gauge the size to be cut, lay out all the parts and then measure the full length and width of what the finished piece will be. Add ¼" to the length and width.
Pro Tip: Thinking ahead can save time and work later. The squares used here are larger than needed because the triangle corners were needed to make leaves for another project.