iMac 27 ram upgrade configuration

I have an imac 27 mid 2011. I have the machine laying on its screen and have access to the memory banks. I have 2x4 gb and 2x8 gb ram sticks to go in…all the same make and speed…but am very confused as to the best configuration. Some people are saying go by dimm and others are saying go by bank. The thing is, there's no indication or labelling of the memory slots so I am unsure if the banks are arranged horizontal or vertical. If someone could advise me what slots to install each ram stick into (with the machine orientated laying on its screen) that would be great! Also, do I need to be concerned about memory cross threading / processing? Thanks in advance!