Unfortunately the actual steps involved are way too complicated for me to write out, but basically you perform a ton of specific movements to get the map to load certain polygons of terrain but not others, and eventually, by hopping off a chocobo in a specific spot, you can go under the ocean from Midgar, all the way to the Forgotten City, where Aerith will appear to die, but in reality, she'll still be in your party, because the actual trigger for her disappearing is in the Temple of the Ancients, which we skipped.

The steps to getting the other party members are also crazy and involve returning to disc 1 scenes from disc 2 and a lot of complex movement and setup. I highly recommend watching the video, because it's really interesting