Its actually very easy,
Install all drivers, then on the DJM 850 check if the output channels are marked as:
usb 1/2 - channel 1
usb 3/4 - channel 2
usb 5/6 - channel 3
usb 7/8 - channel 4
on mixer output you can select all as "none" or one of the channels as "rec out" if you want to record your set on traktor
on traktor, select "external mixer" and route your 2 channels to the desired channels on the DJM 850.
If you start traktor with the controler and the mixer plugged, you can follow the wizard to set up. if the wizard dont pop up on start, you can acess the configurations on traktor then click on the "setup wizard".
My personal setup is 2 control X1 MK1, Kontrol F1 and DJM 850
good luck!