I'm not a master...but I've cut a few up.
Well you really should get some 1/8 fine line tape and a sharp razor saw...although some will say to stack cut off wheels and make 1 cut.
after you figgure out how much you want cut work slowly on your tape line..keep it straight and level...side to side also! As far as adding filler to the roof it will vary depending on where you cut. (jimmy flintstone makes some pre chopped bodies.)
to lower it ...how low do you want to go??? That needs to be answered before you pick a technique...You can drop the axle, reverse springs and cut perches, and of course Z the frame!
Channeling the body is usually just cutting firewall....now if you want an interior...there is your work!
Don't worry its only plastic...repeat this 5 or 6 times then pick up your Zona and cut that puppy!!!
Remember if you cut it Tenax and Evergreen can fill it up!!
So Abe , no simple answer....Plan it and Cut it up and then fix it up......SIMPLE!!!
Moderate Z front and back:
Major Z front and rear:
Note ...this one was also channeled and a new floor pan, and firewall were made, seats are always a fight along with steering!