If you have never tried one, or are not familiar with this mute, it works on all high treble strings, and the upper portion of the tenor section where the strings are spaced tightly enough for it to remain held in place. Whenever possible, the mute should be parallel to the floor, inserted just above the hammer return rail, between two hammer shanks that are slightly to the right of the hammer for the note being tuned (or left, if you are doing the top few notes and coming in from the left side).
To mute just the left wire of a unison so the center and right ring, simply insert it (closed) between the left wire and the right wire of the adjacent note to the left. (Do the same on the other side to mute the right.) To mute two adjacent wires of the same unison, insert it (closed) between those two wires as you would a rubber wedge, allowing the third to ring. To mute the two outer wires, allowing only the center to ring, open the mute slightly so the tips go around the center wire, but still inside of the left and right. When you let go, the tips will spring outward to press against the left and right wires, muting them.