The original signs were likely done with a silk screen. I've hand lettered quite a few of them. Really, there isn't anything different than lettering over a pounce pattern on a flat surface. The brush skills are exactly the same. You'll probably have to double coat most signs needing white lettering. Depending on the size, you'll be using a quill lettering brush and lettering enamels.
If you are brand new to hand lettering, I'd suggest checking out YouTube videos on hand lettering. Even better, find an upcoming Letterheads meeting and go to it. Nothing beats watching a good hand letter working on a project. There are several links to hand lettering techniques on the tab at the top of this forum. Click "The Letterheads Site". Click the link: Lettering Basics and then check out all three pages. http://www.theletterheads.com/lhparts/lettering.html
Feel free to ask specific questions here.