Here is my blog post on this, I'm only putting in the link and a very, very short summary here...surrealisticreflections.blogsp…
Many thanks to TubaMirum and many other deviants here that have encouraged me to share how I do this with others!
Prepare the glass surface so that the paint will stick evenly and stay.
Create the bare skeleton of under drawing with an application of sparse, liquid leading lines.
Apply the initial underpainting to cover the surface evenly.
Apply shading, texturing and colour changing coats of glass paints on top of the underpainting. (Might let some of the underpainting dry for these or not, depending on the desired effects.)
Let all of this dry overnight so you can see the final colours. At this point continue with more coats for further refinement in select areas or go to the next step.
Apply another, thicker and more detailed layer of liquid leading to smooth out the lines, apply thinner, more detailed lines, and cover any bubbles or changes in image overall composition.
Repeat any steps that are required until you are happy with it. You can also cut out offending areas of the painting if you really want to with a razor blade and secure them to the background with leading and glass paint to change or repair an existing painting. If you do that and have a hole in your painting, just treat that area as a miniature of a whole glass painting and follow all the steps again until you are satisfied.
Once you have completed the painting, let it dry a week and then varnish it with glass painting varnish (2 coats) and prepare to hang it where ever you want.