I want a body with multiple layers of different materials that I designed.
trying to answer you question, you need to open the material library by clicking on RMAT command or by going to the render tab.
then, in the search button, look for the material of your choice and add it to your library by clicking on the upward pointed allow on your material of choice.
do this to several materials and then when you are done, close the window.
now go back to the material tab and choose the option in which you add materials by layers, this opens a dialog box in which you just drag and drop the material in your desired layer.
i hope this answer will shine some light on your problem.