Where do i put postage/write the address on the priority mail envelope? read on?

might sound stupid i but don't want to mess up. is it the the blue/eagle side or the back where u have somehting written. i ume its the blue side but i've never seen postage on a non-white background. and 2nd i cant write the address on the blue side (blue pen) or the white side on the backside because there's like 6 lines of information about how to use the envelope etc.