A base coat of paint will turn your wall into a canvas for your artist's techniques. If your wall is already white or off-white, skip this. If not, apply a base coat of off-white acrylic wall paint using a roller and let it dry before you start. If the wall was dark in color to begin with or the paint coverage doesn't seem solid or flat enough, apply a second coat.
Drafting the Image
If you can draw the stained glass image you want by hand in pencil, do that. If you don't feel comfortable enough with your artistic skills for this, look for stained glass painting stencils in a craft store. Alternatively, get a stained glass image from a book or website printed as a transparency at a print shop and project the image onto the wall using an overhead projector. Trace the image in pencil while it's projected. Once you like the pencil drawing, paint over it in black acrylic artist's paint using a detail brush. Leave the black acrylic paint undiluted; the thicker it is, the better, because this will give the finished product dimension and opacity. Let the black paint dry.
Stained glass is quasitransclucent, and its colors are distributed with uneven, marble-like variations in depth. To create this effect, mix each of the acrylic paint colors you want into a semitransclucent solution of one part paint, two parts clear acrylic medium; you can also use water, but the paint is more challenging to work with this way. Apply the translucent paint in layers; let each dry before you add the next. Cover the remaining paint and let the paint on the wall dry.
Creating Light Effects
To make the paint look like shiny glass, use white paint to create spots of reflection. Observe where the lights in your room naturally hit the painting; if you're working in a room with lots of natural light, do this on a sunny day and at a time when the sun is at the brightest in that room. Once you know where the light hits, apply a mixture of thinned white paint to the spots on the painting where the light hits. Paint over both the colored paint and black outline. Getting this right takes practice, but with acrylic you can wipe the white paint off the colored paint and try again.
If you want to give your finished, fake stained glass extra shine, apply at least two coats of clear acrylic glaze or glossy spray varnish to your finished "glass" design. If you get bubbles in the glaze, though, wipe them away; these ruin the illusion.