tgrendl:
Topping on the 212 / 412 can be done in flight, or on the ground to a lower value using a part-power trim stop.
I'm not a huge fan of the in-flight check as it requires one to pull into the transient Ng range (101.5, 102.4 or 103.4 depending on the set-up) and that gives me the willies. As I said above, the later engines like the 3D in the 412EP cannot be topped in flight, and must use the trim, as the max Ng is a screaming 109.2, and you can't get there without getting you name in the book.
At any rate, to answer your question.....basically you roll one back to idle, and increase power on the other until the topping value is reached, ensuring no other limits (Q, ITT) are exceeded. At the target value, the rotor should droop with no further Ng speed increase. If the rotor doesn't droop, or droops too early, adjustment is required.
On the 212 / 412, topping serves both sides of the power equation.... it ensures the power is available for OEI events, and protects the engine from Ng overspeed.
Hope that answers your question.