Just take it little by litte.
Try the stress tests at the bottom of the tutorial and see how things go. Best thing when overclocking is to be patient.
From the bottom of the tutorial:
"If you faced issues in the stress test, start lowering the clock of the GPU memory by -10 Mhz and the GPU clock by -5 Mhz until you find a completely stable overclock.
If your current overclock can handle the hours of stress test without issues it's time to do the opposite and increase the GPU memory by +10 Mhz at a time and GPU clock by + 5 Mhz at a time to find the perfect stable overclock.
Keep in mind to give yourself some headroom. If your maximum stable overclock was "+200 Mhz GPU clock" and "+350 Mhz GPU memory" i would recommend something like "+195 Mhz GPU Clock" and "+340 Mhz GPU memory".
Even if you stress test for hours with no issues the GPU may crash once in a while, this means that you need to decrease the GPU memory or GPU clock. You can play with those settings."