Some of this has already been said, but I spent a lot of time writing this, so I'm posting it anyway, damn it.
First of all, you have to be running the Slavery civic.
Second, your city has to have enough population. You can only sacrifice a maximum of 1/2 of the city's population. So if 3 pop are required to whip a build to completion and your city is size 5, you'll either have to wait until the the city grows to 6 or enough hammers are in the build so only 2 pop are required, whichever comes first.
Finally, in terms of the on-screen specifics:
As you can see, towards the lower right, below the DRAFT button is the "pop rush button" (on the left; the button to the right, also immediately below the DRAFT button, is the gold rush button, which requires the Universal Suffrage civic). Click on it and Bob's your uncle.
By the way, the normal +1 unhappiness from whipping is not 20 turns; this game was played on epic speed, so it's 15 turns (10 on normal speed). I had whipped in this city 10 turns earlier, you see.
Now as for strategies on how to use whipping most effectively, look around the board, especially the War Academy. In a nutshell, on normal speed with no hammer multipliers (such as forges and/or Organized Religion), whipping yields 30 hammers per pop point. On epic speed it's 44 hammers, IIRC. The best time to whip is when 2 pop are needed, but only just. In other words, when there's just a little over 30 hammers still needed for the build. The reason is that the additional hammers are overflow and go into the next build, accelerating it as well. Cool, huh?
Remember also that Granaries will help you recover that population you whipped away faster.
Enjoy your tyranny!
