Need Tactic Help please

So i am playing about with new tactic system but am confused . I set my team up to  4-3-3 at the default screen , I then go to custom to set up my desired in game tactic and player position, but because they are in the good position in the default screen, i move them in the custom screen and they are half a team in bad area on pitch , . How am i supposed to get a good default tactic with good chem and then a good custom set up with same players