its one of the hardest parts tbh, for synths & melodies, a few pin point notches can be most useful in the low-midrange, for basslines, sumtimes wide boosts are better, for eg you might want to do a 6db+ boost ~80-90hz, and 800-1.5khz, to bring out the bass and midrange, then take a few pin point notches out around 100-250 for the snare and kick to get thru the mix, or maybe a shallow reduction of the 180-450hz area which can be where muddyness really messes with the mix....numbers are very rough, dont take them as absolute, experiment with them and see what happens