Step 4: Paint the Plexiglas
Edit: If you want to make life easier you can start with opaque white Plexiglas but then you probably won't be able to see the symbol when the light is off. Either way you'll need to sand the Plexiglas prior to painting to ensure that it will stick.
Sand the Plexiglas on one side, make sure to do this very thoroughly, it will help to keep the paint from coming off. Doing this with a rotary tool or fine grit sandpaper will probably turn out better than trying to do it with the coarse grit paper that I used. Once thoroughly sanded, align and apply your stencil, this may take a couple tries to get it just where you want it but its worth taking the time. Just remember that if you put the stencil on wrong peel it off slowly and carefully to avoid ripping it or leaving pieces of the adhesive side behind.
Once that's finished, begin cutting the black parts out of the stencil with your X-acto knife, I used #11 blades for this. They work a little better for this fine detail work than some of the other blades I've used, and they are relatively inexpensive.
If your picture is pixelated (like mine) there should be rather distinct layers of shading around the black parts. If you start cutting on one level of the shading make sure you continue along the same line or your stencil will have uneven and unintentionally curvy lines. When painted, this will be unnoticeable from a distance but up close it will show a little bit. Once you have the stencil cut out, lift up the edge of each of the black parts with the tip of your x-acto knife and peel them out.
Now tape some masking to the other side of the Plexiglas. This can be as simple as a piece of paper but is necessary if you want to keep from getting paint on the other side of the glass. Start by painting the black paint on the stencil; after doing the first layer of paint hold it up to a light source. If you see little pinpricks of light poking through the paint do another layer and check it again. Any little holes will leak white paint through and your warning sign will have mini polka dots. After the black paint is thoroughly dry slowly remove the stencil. Next, paint the white over top of the black paint. This only needs a thin coat, thick enough to not leave any holes but thin enough so that light still easily passes through, this is most easily achieved with spray paint. Once dried it's time to move onto the next step.