Computer program. I think it's possible to draw a project and scan it but there will be graphic corrections to be done in the software.
The problem with custom plaque is that you do not see the " marble " effect (or other effect) on your project, the design will be adjusted at the factory, but there will be losses or too bright colors, etc.. and it will be necessary to correct.
Without concept of graphics you will have trouble. You can send a logo to the factory and they will do the work for you, but if you want a really custom result it will make the model yourself and adjust with sample. For example, at first I chose the GOLD option for laser printing characters, Serious error ! It looked like a business card from a chinese restaurant, really not great it's much better in silver. :] Etc..