The last component to install onto the motherboard before installing it into the case is the RAM. Start by locating the empty RAM slots on the motherboard. The vast majority of motherboards will have DDR2 RAM slots, there should be at least two slots, with mid to high ending motherboards having four. Located on either end of the RAM slot are retaining clips that will hold the RAM in place, open these by pushing them in opposite directions away from the center of the slot.

Then with both hands pick up the RAM memory module bit it's edges and line it up with the socket so the groved portion of memory lines up with the notch in the socket. It only fits one way so you'll want to make sure you have this correct before pushing it down into the slot. When you're confident you have the RAM chip lined up properly push down on both ends until the retaining clips snap into place.
Repeat this process for as many RAM memory modules that you're installing.