WARNING: This is where the most delicate parts are. Be extremely careful here!

If you look in the middle of the assembly, you'll notice there are four little metal tabs holding the top and bottom together. Use the slotted precision screwdriver to gently unclip each tab. You will need to apply a gentle separation force to prevent the tabs from re-engaging when you move on to the next tab.

NOTE: The picture here showing three of the metal tabs was taken after the two encoder halves were separated.