Having fastened the appropriate brackets to the heatsink’s sole, you should then secure the heatsink on the CPU with two light spring-loaded screws.

And finally you have to put a fan down on the heatsink:

As you see, there’s no obstacle to mounting a Noctua NH-U9 on an ABIT AA8-DuraMAX mainboard, but if you tried to install the larger Noctua NH-U12 (which is 29 millimeters wider), the chipset cooler would get in your way. You can avoid this difficulty by simply turning the chipset’s cooler round so that its airflow was directed towards the CPU socket. I can’t say how compatible the Noctua NH-12U is with other mainboards, but the distance between the cooler’s sole and the bottommost plate of the heatsink is 32 millimeters. I guess this information may be useful to you.

To test the Noctua NH-U9 I used a 92mm Coolink fan (model 922) with a speed controller and a speed range of 1100-1850rpm (this is what the specs tell you, but the maximum speed of this fan proved to be about 2000rpm in practice). The performance of the fan varies within a range of 42 to 64CFM.

The Noctua NH-U12 was tested with a 120mm fan from Thermaltake (Thunderblade A1926) with a speed controller from the Coolink fan (a speed range of 1100-2000rpm and a performance of 42-78CFM).