Personally, I used the template provided with the model. If there's none in the files, try to download other skins for the vehicle you're interested in. Hopefully, you'll be lucky enough to find a template included in the download. If not, you can edit the png itself, using it as a model to produce your own skin. Also, be sure the model you're working on is already mapped so that your skin can be displayed correctly. So far, I haven't touched to the ambulance model, I don't know if there's any templated ambulance.