Install SetCPU from the market. When you open it if a pop-up shows, read and say "OK" or whatever. You'll have two buttons on the home page. Click the top one, which should say something like auto-detect.
When it's done, and if you have an OC kernel installed, you'll be able to drag the slider right or left to set the max speed. Try dragging the slider all the way to the right and try using it at that speed to see if it's stable enough for you. If you get frequent reboots, then drag the slider a little to the left until you reach a speed (800 is stock) at which your device is stable. Once you have that speed, click the "set on boot" button to have this and any profiles (I'll get into that in a second) automatically apply every time your NC boots.
Go the the Profiles tab. Profiles allow you to set min and max speeds based on certain conditions (charging, sleep, varying battery levels). You can also set priorities for the profiles. Click on the add profile button and select the settings.
I use a sleep profile that maxes the speed at 600 (always leave min at 300) when the device sleeps. This helps maximize battery life. I also use a profile to max speed at 800 when it's charging. I only do this because past devices have had trouble charging at non-stock speeds. I also have a couple of profiles that gradually lower the max speed as battery level decreases. After you've created profiles click the Enable box(es).
On the home page and on all of the profiles I use "on demand", because it will use the lowest speed necessary at any given time and only overclocks when the system needs to do so.
Overclocking will drain battery faster, which is a good reason to use "on demand" and profiles. Overclocking for long periods can cause the CPU and battery temperature to rise, which could damage or reduce the life of your device. In most cases, it's not a problem, but, in most cases the device has an internal guage that detects the temperature of the CPU and/or battery. On my other devices I had a SetCPU profile that would underclock the device if it got to hot. Can't do this on the NC (yet).