"but if one relaxes, works with a motion that doesn't change and has his body moving to the spot he wants to hit, he will be successful more often than not."
well said. thats one of the keys with any off-speed pitch. keep the same delivery, same motion, same arm speed, same arm angle as your fastball.
if you can be consistent with all that, so every pitch is coming out of the same spot from the hitter's perspective, you'll be in much better shape. they'll have a tougher time picking it up. and that makes your job a lot easier.
as for where to start your curve to get it to break where you want: experiment. find what works for you. koufax used to imagine bucket behind homeplate, and tried to get the ball to dive into the bucket.
it depends on your break, and the angle of your curve, 12-6, 1-7, 2-8, etc.
ive got a 1-7. if i want to throw one for a strike, i generally key in on a RH batters shoulder and it will break over thigh high. if i want him to chase it, i start it belt high and slightly inside and it breaks down out of the zone.
as other have mentioned, it takes practice. phsyical and mental. ive thrown a lot (LOT) of bullpens over the years, and thats when you get the muscle memory u need to make these sorts of adjustments automatic.
part of it is mental/visual. try to actyually see the path of the ball before you make a pitch. see it leave your hand perfectly, breaking exactly the way you want it. see the hitter reacting just the way you want. see the ball hittting the catchers glove in just the right spot.
then, relax and trust that your body knows how to make that pitch.
keep working. it takes practice.