Switches (not the switchplate) are mounted in a rectangle or square box that is mounted in the wall. With the switchplate removed, you'll notice mounting screws on the top and bottom of the switch for mounting into the electrical box. Loosen the screws just enough so the switch can move back and forth. You'll notice at the top and bottom of each switch there is about 1/4 " of play at each mounting screw. That means, for a double toggle, there is 1/2" of play.

With the screws still loose a tiny bit, place the switchplate over the switches and adjust the switches accordingly so they line up with the switchplate. Start ALL the screws first which continues the lining up process. Tighten the screws like you would a tire on a car--alternate. Remember, some switchplates are sturdy but may break due to over tightening. Do Not Overtighten...just firm is good enough.