Install a template in a mod?
I guess you mean installing an edited template: a texture.
Open Spark IV, open vehicles.img, and go to the vehicle you want to edit. Select and open the .wtd file of that vehicle -for example "police.wtd", and find and select the texture you want to replace (it is usually called "paintjob_sign", "police_sign", or something so). Then, click on import texture and select the new texture you want to add. After importing it, click on "save and close" (the .wtd will close) and save again.
If you mean how to create a template, that is made with a modeling program, but I don't know more
I hope this helped you