There is really not an easy way. Even if you take the converter out, most of them have that bulge in the tranny side that holds fluid. You can tilt it and mess with it but sometimes its like trying to get all the water out of a tire
Of course, that will get al but about a pint so no big deal. I've heard good and bad things about having it professionally flushed. Many Kwiky Lube places have the little cart with the fancy piston that supplies new fluid for about $70. It certainly gets all the bad stuff out, but some guys are of the opinion that this method isn't wise and should be avoided. Maybe someone with some info can speak up on that.