First of all do not mess with voltage.
Now for the tutorial. Start increasing core clock +10 then run some heavy benchmarks for example heaven vally if its stable without any artifact add +10 and continue doing that . When you get artifact decrease your clock speed -5 (beware that nvidia will reset your clock speed in case of crash so make sure to apply them aigain) also increse your memory clock set it to something like 50 it doesnt really matter that much.
Anyway with gtx 750 i dont thing you will reach more than 50mhz on core clock but it always worth trying ---Dont mess with voltage if you want to be safe!!---