Download MSI Afterburner and install it.
Download Unigine Valley benchmark and install it.
Go into afterburner, increase max power by 10%, increase core clock by 20MHz.
Run Valley Benchmark.
Repeat until Valley Benchmark crashes.
At this point, you should decide whether you want to play with the voltage or not to get even better clocks.
If not, just fine-tune the core clock until you reach the highest one that Valley Benchmark will complete with, then run the benchmark for about an hour. If it doesn't crash, repeat this process with the memory clock.
If you want to play with the voltage, raise the voltage by 10mV and continue overclocking the core clock until Valley Benchmark crashes again, and raise the voltage again. I wouldn't go...