I'm no "whipper" anymore, but in my younger MX days...
As said, you have to feel it. That said, a whip is innitiated by kind of starting to turn on the face of the jump, the bike will want to whip on it's own (kicking the rearend out) and you use the bars and your legs to get things lined back up before landing. Find a good airtime jump and work your way up to more and more radical whips.
Here's some WAY old school... We called 'em "crossups" or even "Decoster's" (he started it as a way of setting up for a turn, while today's "scrubs" are intended to scrub off speed and get back to the ground and accelerate sooner).