You can easily add patterns or shapes to windows and mirrors using a simple DIY stencil and some cheap paint. Design blog Design*Sponge posts a series of projects centered around using easy, home-brewed stencils to add visual interest to your home. The process they use for mirrors and glass windows is pretty neat: you apply a sheet of contact paper to the glass, smooth it out and draw the pattern you want onto the contact paper and then trace the pattern lightly with an exacto knife. Peel the inside of the pattern out and you've got a stencil adhered right onto the glass you can colour in with some basic latex or acrylic paint. You can even go for an etched look without damaging the glass by using clear acrylic glaze instead of coloured paint. For more detailed instructions, check out the full tutorial at the link below.