Just sit down with a piece of paper and draw up the two formations on top of one another. You'll see, for example, that when the opponent is playing a 4-4-2, he only has two central midfielders, so if you play a 4-3-3 with three central midfielders, you can outnumber him in the middle and control the ball easily. Or, if you're playing a strong 4-2-3-1 team, you can use a 4-3-3 so that your midfield triangle matches the opposition's, so that there's always someone there to close down each of their midfielders and track them toward the goal. Then, at the back, you have to think about things like how your number of centre-backs matches up with the opposition's number of forwards; that is, if they play two forwards and you're in a four-man defense, you don't have an extra man at the back to match up with them, whereas if you played a three-man back line, you'd have two man markers and a sweeper (this is why, for example, Greece at Euro 2004 changed from a 3 to a 4 man defense based on the opposition's forwards; they kept a spare man at the back). So yeah, like I was saying, you just want to sit down with a pen and some paper and put some thought into the way that different formations match up against one another, and you'll be able to figure out what you want to play against a given look from the opposition.
