I finish the sign completely first. I paint the sign a similar color to the smalt, so black if you are using black. The letters are leafed or painted, every thing is done. Make sure the paint is well cured.
Then I lightly scuff where the smalt goes. If it's a large sign the smalt is done in sections. I hand letter the smalt "glue", I guess you could use a mask. Stay away from all edges about a 1/6-1/8".
Apply a good heavy coat.
I use a coffee can with holes drilled in the bottom to apply the smalt. Cover well, I pat it down lightly with my hand. After it dries for a day or two flip onto a clean piece of paper so you can recover
the smalt that doesn't stick. Carefully brush off any pieces that stuck where they don't belong.
These days the issue is what to use as the glue. I had used Holdfast but haven't been able to get it for a while. Now I've been using Smiths cream. Either way you mix some one shot same color as the smalt with it. I've heard of some people using West epoxy which sounds like a good ides.