(Assuming you've already turned off the power supply and opened up the case...)
Step 1 - ground yourself to the system to prevent static discharge.
Step 2 - figure out which slots to install the RAM in. Your motherboard manual is useful here. If there are different coloured slots then fill all slots of the same colour first before adding RAM to the other slots.
Step 3 - flip back the white tabs at each end of the slot so they're not in the way as you insert the RAM.
Step 4 - Look for the notch in the socket and orient the RAM so that it's bottom notch aligns with it.
Step 5 - Lower the RAM straight down into the socket. When it makes contact, push down very firmly on the edge of the module with your fingers, distributing the force across it's whole length as best you can. It takes a fair bit of force, and when the module seats itself the white tabs will flip up on their own and engage the end notches on the RAM module.
Use a bit of caution here - if you've got the wrong type of module (Registered instead of Unregistered memory, for example) the bottom notch may LOOK like it lines up with the one in the socket but in fact it's very slightly offset to PREVENT you from inserting the module into the socket. If the module won't go in with a fair bit of force then look very closely at the bottom notch, perhaps removing one of the existing RAM modules to check it against.
Step 6 - Ensure that the white tabs at each end of the socket have fully engaged into the retaining notches at each end of the module.