You can attempt to overrun as an attack action made during your move action, or as part of a charge. (In general, you can’t make an Attack Action during a Move Action – this is an exception.) In either case, you don’t get a 5-foot step before, during, or after the Overrun attempt. With an overrun you attempt to plow past or move your opponent (and move through the area.) You can only overrun an opponent who is one size smaller, the same size as, or one size category larger than you.
First, you must move at least 10 feet in a straight line into the target’s square (provoking attacks of opportunity normally).
Then the target chooses either to avoid you or to block you. If the target avoids you, you keep moving. (You can always move through a square occupied by someone who lets you by.) If the target blocks you, make a Trip attack against the target. If you succeed in tripping your opponent, you can continue your movement as usual.
If you fail and are tripped in turn, you fall Prone in the target’s square. If you fail but are not tripped, you have to move 5 feet back the way you came, ending your movement there. If that square is occupied, you fall Prone in that square.