First, the design produced in Adobe Illustrator must be composed in an accurate scale represenation of the full size sign face or display area. The scale must be right or at the very least the aspect ratio must be right. I've seen people home-brew their own designs on a vertical letter size piece of paper and then expect it to transfer over to a horizontally long, rectangular area. Getting the math correct from the outset of 100% critical.
The finished design can be transferred to the target surface using a choice of methods. One is the old-fashioned grid system used by many mural artists. Divide a small master drawing printed at scale into a grid of small squares and then transpose that grid to the full sized target surface, be it a wall, window or sign face.
For some designs (stuff that's just mainly lettering or simple graphics) the artwork can be output from a vinyl plotter/cutter either as cut stencil maskant or as a pounce pattern. Maskant can be painted over directly or in the case of a window be used to lay down grease pencil marks more precisely. A pounce pattern is taped to the target surface (such as a window) and a type of chalk powder is pressed through it. Remove the pattern and the dots of powder are left behind as a guide.
Some people use projectors to throw the image into the final surface. But some care must be taken with that approach. If the projector is throwing an image onto a large target surface, such as a side of a building, and the projector is positioned too close to the target surface the projected image may be optically distorted with various keystone issues. But, sometimes a warped look to the final design might be the intended effect.