Wedge your fingers between the upper case and the lower case, starting from the rear of the MacBook between the hinges.

Lift the lower case only slightly to avoid damaging the cables that connect the lower case to the upper case.

Keeping a firm grip, lift steadily until the lower case separates slightly from the upper case.

You may experience a lot of resistance when lifting the lower case. If necessary, slide an opening pick or other ESD-safe pry tool down the side edges of the lower case to pop the two hidden retaining clips free.

During reassembly, to re-engage the clips, press firmly near both side edges of the lower case (near where the pick is inserted in the third image) until you hear the clips snap into place.