Tuning the KK2 board isn't difficult, but it IS a bit time-consuming to get it just right. Lots of trial & error.
I'd recommend following the instructions in the "PI Gain Setup" file you'll find on the HK site, under the "Files" tab for the KK2 board. Or here's a direct link: http://www.hobbyking.com/hobbyking/s...4X68736X56.pdf
You should set the Roll/Pitch I-Gain to zero before you start. Also set the Height Dampening to zero. Concentrate on tuning just the Roll/Pitch/Yaw P-Gain values first. Once you get a good steady hover by adjusting only the P-Gains, THEN start increasing the I-Gains to get smooth forward flight. Finally, add height dampening (if you want) to help smooth out your throttle inputs.
Once you have all that dialed in, you can go to the Stick Scaling menu and play with those values. Those are sort of like programming the end point adjustments on your transmitter. You can adjust how sensitive the controls are to your liking. Higher values are like having more servo throw; it makes the model more sensitive.
You'll find that there is also an "auto level" capability on the KK board. Leave it turned off until you get everything else right!! It'll just get in your way. If it's on and your quad doesn't behave right, you won't know if it's a gain setting or auto-level messing you up. Best to just eliminate it at first.
And take lots of notes as you go!! Write down every value you set, how it performed, and what props you were using. Different props make a huge difference in how it behaves, and you may want to revert back to some other setup.
Here's a good youtube video that shows what happens with different gain settings: