First read the Apple instruction manual for the dims you will be installing . Shutdown your Mac pro and wait at least 10 minutes for it cool down.
Check the temper of the computer to make sure it is fully cool before starting doing work further here before you open it
Unplug all cables except for the power plug then touch the metal casing to stop any static electricity coming out before you unplug the plug.
To open your Macpro lift the latch on the top and turn it 90degrees to the left now left the housing straight off and set it to the side for now and slide the dim latch to the left to unlock the Dimm covers
Once your cover removed put it to the side for now. Now remove the lower cover of the left DImm side slots
Now you may open the Dimm ejectors on either side of the