These methods can be used for any vehicle, but all the materials are for Conrail in this project.
First, do some research. Google Images will be your greatest asset. Conrailers go to crcyc.railfan.net. Find your car or truck, or a similar unit if its a custom car. Second, open a Microsoft Word document (or whatever word-processing application you have) and paste the logos that correspond to that vehicle (for Conrail, yellow paint=blue decal, blue paint=white decal). Third, size down the logo in the document. To accurately tell if it will fit, click print preview, and magnify to 50%. Resize the logo until it either fits or you’re satisfied with it. Next, have a cutting implement. Exacto-Knives are perfect, scissors get things done, and if you have neither, a sharp-tipped pocketknife is fine (mini-Swiss knives are near-Exacto quality). And if all else fails, a small paring knife is fine. Once you have a cutting tool, get a glue stick or some small double-sided tape. Make sure with the tape that some clear one-sided goes over top. Print out your sheet if it hasn’t already. Cut your sheet with extreme caution: one mess up on a logo, and you repeat the process. If it’s a Conrail logo, cut a square around it. For a CR Police patrol logo, cut the shape out entirely. Apply the gluestick or tape to the BACK of the logo. Place the logo on the location you wish. That’s all there really is to it. You only need a printer, some sort of word-processer, an adhesive, a cutting tool, and a vehicle to do it on. Das it!