You can enamel shapes onto sheet metal or layer an enamel shape on top of an enameled disc. But it can be a challenge to get the stencil shape just where you want it. I suggest cutting a paper pattern the same shape as the piece beneath it, extending one or more edges and cutting the stencil in this pattern

Use a marker to designate the edges of the pattern that will be flush with the piece. After getting the paper pattern wet, lay it on the piece with the marked edges matching.