How do you install a 20 amp switch for well pump?

Black/White/Ground power in and the same out. Tie the incoming and outgoing white wires together under a yellow wire nut and push them back in the box. Tie the ground wires together under a green wire nut and connect the pigtail from those ground wires to the ground screw on the switch. Connect the 2 black wires you have left to the 2 screws on the switch. Doesn't matter which black wire you connect to which screw.