dell poweredge r510 memory changing

Hi! i want to change memory in my dell poweredge r510. i have 4 modules that have 8gb ram, two modules (not the same as 4) with 8 gb of ram, and also other 2 modules with 8 gb of ram too from yet another company. How can i put in these modules to slots to use them all? my server have 2 processors with 1-4a and 1-4b slots.