Turn the mute on its side so that one of the openings is directly on top of the D-string with the ledge facing toward the bridge and the neck of the violin. The ledge is the small piece of rubber extending outward about 1/4-inch from the top of the mute.
Push the D-string into one of two small circular opening on the side of the mute and insert the A-string into the other. Place the mute so the extended part of the ledge faces the bridge.
The mute may need to be pulled down away from the bridge to position the A-string inside the opening.
Move the mute up the strings until it's on top of the bridge to create tension between the strings and the mute, keeping it lodged in place. To remove the mute, slide it down the instrument and pull it off one string at a time.