To create the lamp lens the glass paint needs to be used unthinned. I usually apply it with another pin with the point snipped off. This allows slightly more paint to be carried on the tip.The photo below shows how I keep them vertical, by just pushing the pins into a scrap piece of balsa. A tiny drop of paint is just touched on the heads. Surface tension creates the lens as the paint will dry more or less in the shape that it's applied. ie a dome. It usually takes a couple of applications to achieve the right form. It dries very slowly,so I normally give it at least a day before I touch them.
As best a close up as I can manage is shown below. The paint is clear and reflects the chrome on the pin head and catches the light much like a reflector. There is also the chrome rim of the pinhead giving a nice finishing touch. When fitting up to the car body I normally snip off the head leaving around 5mm of the pin for insertion of a pre-drilled hole. The pin is then fixed from the inside using cyano. Cheap, simple but very effective.