Summarize:

Take the lamp shade off of the fixture. Wash it with warm, soapy water, then dry it off with a towel. Wipe the lamp shade down with rubbing alcohol to remove any oil or residue. Handle the lamp shade from the inside from now on to prevent oil transfer. Do not clean the lamp shade if you already painted it. You can use regular stencils or self-adhesive stencils. If you are using a regular stencil, tape down all 4 edges with painter's tape. If you are using a self-adhesive stencil, peel it off of the backing first, then press it down onto the lamp. Choose an ornate stencil with thin lines so that the light can pass through. Flourishes, filigree, and ornate butterflies make great choices. Squirt some glass paint onto a disposable plate or palette. Dip a foam pouncer into the paint, then tap it against the stencil. Work your way from the outside edges of the stencil towards the middle. You can use glass paint in any finish you want, but be aware that translucent glass paint may not show up well, especially against a previously-painted surface. If you painted your lamp shade with tinted decoupage glue, use acrylic paint. Tinted decoupage glue will be too fluid for this step. Do not let the paint dry in the stencil, or you risk peeling it off. As soon as you finish applying your last stroke of paint, peel the stencil off. Avoid dragging the stencil across the lamp shape; otherwise, the paint may smear. Instead, pick the stencil up by 2 corners and lift it straight up. If you notice any chips or gaps in the paint after you remove the stencil, fill them in using spare paint and a thin, pointed brush. How long the paint takes to dry depends on the type of paint you used. Acrylic paint dries within minutes, but glass paint can take several hours. As the paint is drying, use this time to clean your pouncer and stencil. Wipe your stencil down with rubbing alcohol. If you have a self-adhesive stencil, be careful not to get anything on the back. Wash pounces using soapy water or brush cleaner. Be aware that it may not be salvageable and you may have to throw it out. At this point, you can repeat the process to apply more stencils for a layered look. You can also apply stencils to other parts of the lamp shade. Let the paint dry after you peel the stencil away. For example: If you added 1 big butterfly, you might want to add 1 or 2 smaller butterflies. If you added a filigree heart, a flourish to either side of it might look pretty. If you added a flower in 1 color, consider adding 2 more flowers in other colors. Once the paint has completely dried and cured, you can reassemble the lamp. If the paint still feels tacky, it has not finished curing. Give it a few more hours; double-check the label for more complete drying times.
Remove the lamp shade and clean it. Adhere a stencil over the lamp shade. Apply glass paint over the stencil with a pouncer. Peel the stencil off before the paint dries. Allow the paint to dry and clean your supplies. Apply more stencils, if desired. Reassemble the lamp.