My husband had two ten dollar plastic lamp shades and I decorated them two ways. One was with felt-tipped markers. To begin with, I just outlined the pattern I wanted with non-permanent markers,then when it was perfect, covered it with permanent marker. I let it dry for half an hour, then carefully wiped away the non-permanent lines with tissues. I also smeared on some watercolours to make the sky. It will never dry and will come off in your hands but so what- how often do you touch the inside of a lampshade?
Here's the one I decorated this way:
My husband liked this and wanted me to make another, but one with geometric designs. Ideally he wanted a deltoid shape, but that was hard on a curved surface, so I did something else instead. I cut up some fish net stockings, stretched the opening to make a triangle, and fastened it with clothes pegs. Then I took the rest of the piece of stocking, and brought it up behind the triangle and fastened that with clothespegs as well. Once I had done this three times, and added three more loops for decoration, I tied everything with black elastic string at the back and got this result:
Pretty cool, eh?