Where you are going to mount the panel is the first step.
I wanted the plugs to be accessible but not obtrusive. I chose the floor console right below the drink holders. this may be a bad idea for people who use the drink holders a lot because you could spill liquid into the plugs causing a short. I don't use them much so this is where I chose to put my panel.
Once you are firm on a location, you need to design the layout and shape of your panel. I used a program called solidworks to do this but the freely available google sketchup will work fine.