For Overclocking your GPU you start to bump up Core, Memory, clocks by low value, say (50 MHz) then you test your OC stability (play games and make load on GPU)
if no problems happen (crashes,freezes, BSODs) then you're fine and you can bump those a little bit higher.
Repeat it many times until you reach the limits of your OC (until you crash or you get BSOD)
Note 1: Check temperatures after each OC you do.
Note 2: Don't mess up with voltages as long as you're stable. IF you're unstable bump up voltages a little bit high but i recommend you to ask first.