If there are two slots then add to slot 1 first then 2 or two sticks. It's better if they are the same size sticks 2GB and 2Gb or 4GB and 4GB etc but it will still run with mismatched sticks. Keep them the same speed or the board will downclock to the slower speed. If it's 4 slots it's better to add them to every other slot before pairing them up because each pair should have a separate DMA controller