I think a easy way would be to set a message string and timer (integer or single). When you reached a certain score, you can set the message to what ever you want and set the timer to x seconds (or x millisecond).
Next you can render the message if Timer > 0 in the game main render loop. In the update loop (can be same as render loop) you decrease the your timer by the time that is passed until it reaches 0.

(This is the theory)