[cbk1994]

What do you have so far? Post some code.

Probably the simplest approach is keep track of the player's current position in a combo. When they hit the right key, increase the position. When they hit a wrong key, reset their position. If they hit the end of the combo, they've hit all the keys in the right order, so do whatever action.

But there are lots of potential approaches here, so post some code so we know where you're getting stuck.