-I'm not a pro, but I did stay in a Holiday Inn once-
Like Oosik says, start chunking at the top of the tree.
If weight is being supported by top of tree, most of your cuts need to made upwards (bottom-to-top) through log. This is harder than cutting downwards. Or, if cutting downwards, use wedges so trunk pinches wedge and not saw bar.
Think through and pay attention if 'kerf' is opening up or pinching down before saw gets completely hung up. Have more wedges and a big hammer nearby for miscalculations.
As you chunk up the top, if shredded "hinge" is weak enough to keep bending, the new "end" you create at the top with each cut will continue to rest on ground as trunk gets shorter, and trunk's angle gets steeper.
If eventually only the hinge end supports the whole shortened trunk (top end is off ground), you can weaken the hinge, or create a new (proper) one below it, by backcutting just a little at a time (i.e. not all the way through) to pivot trunk more to keep lowering the top in a controlled manner; or if you feel safe enough, to eventually cut trunk completely off at stump.