What do you mean by "work properly"? If it isn't sealing the sink drain, it probably just needs adjusted under the sink, on the back side of the drain pipe. You will see the rod that operates the stopper (up and down) with a metal clip on it. Squeeze the clip (to release) and pull the rod down until it stops. This should also pull down the rod coming from the drain pipe which opens the stopper. Release the clip and you should be good to go. May take some minor adjustments but is a quick fix.
if you have never made a mistake, you probably haven't done much.