where should I place the terminations resistors for RS485?
I think this picture in the guide from MAXIM is self-explanatory: -


one resistor at one end of the common shared multipoint cable and one resistor at the other end.
don't use star-cabling techniques - the multi-drop cable has to be used and physically, it can snake this way or that way but keep tee-off point lengths to a minimum.

It's all about preventing data corruptions due to fast data signals reflecting from the open end termination of the cable.