You just need to leave the part, wish to be shown as transparent, as white or leave it without background color. Because there is no white ink in offset printing, any area left white will remain unprinted revealing the actual stock.
If you will require white ink printing, kindly ensure to select for CLEAR+WHITE plastic. We will then require two files from you;
File 1: CMYK Printing File – This will be the complete design with all the CMYK values. White areas on this design will be left unprinted.
File 2: White ink areas - This will be the area for white ink. Areas for white/backed in white should be denoted in black. You can play with different opacities with the white ink.
Example final product: