Material list
1. Etching acid (from local craft store),   2. Protective Gear: gloves, masks and eye protection ,   3. Water,   4. Mirror,   5. Paint brush,   6. Carbon paper,   7. Craft knife,   8. Cutting board,   9. Pen or pencil,   10. Print-out of chosen design,   11. Sheet of adhesive plastic.

A few pics followed by the write-up

Step 1. Transfer design onto adhesive plastic sheet
Using carbon paper and a pen or sharp pencil, trace over your design onto the backing paper of the adhesive plastic sheet.

Note: You are tracing on the reverse side, so make sure your design is the opposite way to how you wish it to be on your mirror.

Step 2. Cut out stencil
Using a sharp craft knife and a cutting board, cut out the design to create your stencil.

Step 3. Stick adhesive plastic in position on your mirror
Peel off the backing paper on your adhesive plastic sheet and ensuring your cut-out stencil is in the correct position, stick in place on the mirror. Be sure to allow a lot of excess plastic paper so you have plenty of room to apply your etching acid without affecting other parts of the mirror's surface.

Step 4. Applying the etching acid
WARNING: This etching acid is extremely bad for you in many ways. ALWAYS keep hidden away in a high place away from children and take special care when using it. Make sure you have plenty of water to wash away excess but please don't wash off directly into a drain! Thoroughly diluting it and washing into the soil is the safest way - well away from your precious plants, of course! Wear all the protective gear you can, and when using outside, make sure it is not windy.

Read the instructions on the etching acid and follow the directions exactly. Pour etching acid over your stencil and thoroughly cover all the cut-out area. We poured it on then brushed a thick coat over our stencil. Then we allowed it to sit for about ten minutes. The acid eats away at the surface of the mirror but does not affect any plastic surface.

Step 5. Wash off etching acid and reveal the effect
Using plenty of water, wash away the acid into the soil. Once all acid has thoroughly been washed away you can peel off your stencil. Dry the mirror and see what you have created!

Be sure to dispose of your gloves properly, as well as anything else that may have the acid on it.