MSR - AKA "Mirror Silvering Remover" is a DIY friendly way to do artwork on mirror using stencils.
Using commercial mirror (not hobby lobby, IKEA or other low priced mirror) use a QUALITY paint stripper on the back to expose the raw silver backing. You can softly clean the residue left over with a paper towel and water. SUGGESTION: USE HOME DEPOT AIRCRAFT PAINT STRIPPER.
Apply stencil to the raw mirror (stencil must be cut in reverse since you are working from the back of the mirror.
Using a cotton ball for larger area and a q-tip for finer detail carefully apply the msr first in a dabbing motion and allow to sit for 20-30 seconds and then wipe with the same cotton ball and the silver will start to disappear leaving clear glass.
Once the backing paint is removed you can do all kinds or artwork to the open areas such as gilding, painting and more. Since this will be framed art you can use a shellac to the back of the mirror (spray or roll on), followed by whatever color of paint. Best results spray 2-3 coats allowing 30 minutes to dry between coats and on the last coat allow 24 hours to cure.
SAFETY FIRST: Always keep all chemicals away from children and pets. Wear proper eye gear and rubber/latex gloves to protect your hands from chemical absorption. Finally a respirator to protect your lungs. While MSR is a DIY friendly product there are still dangers to any chemical.