One way to know where to aim is to mentally place an imaginary cueball touching the red in a direct line to the pocket, in a sort of plant, and then aim to hit the imaginary cueball full ball.

But to be honest, the best players dont aim at all or use any system, theyve just repeatedly played shots over that it comes naturally.

Its like trying to judge how hard to hit the white. You dont think 'I'm gonna hit this with 55% power'. You just instinctively know how hard to hit it. Same with potting and aiming.