I've had them open many many times (so many times that I no longer feel it's a pain to do so and I no longer have problem putting them back together).
As Greebo said, the important thing is to have the pedals in the "free". You just wanna turn the entire unit upside down, and rest the 4 corners on to something.
I use a upside down stool for this (4 legs perfectly spaced for the corners of the unit).
There are two springs in there which quite easily fall off (springs connected to the yaw axis of the pedals).
When turning it upside down, make sure you have something "behind" the unit so that you can put away the top half before disconnecting any wires, otherwise you really will need that third hand.
Most difficult thing when putting them back together again is getting the pot pin centered so that it actually responds again, hard to exlain what it looks like. If you run into trouble, PM me and let me know if you need pictures of it.
Good Luck!