1) Unscrew the three screws on the side of the X1204USB and remove the panel (keep these screws).
2) Repeat this process with the other side.
3) Set one of the rack ears flush against side of mixer so the holes line up for screws and front panel (for rack install) is facing the top side of mixer.
4) While holding ears, tighten the screws into the mixer's left side screw holes.
5) Repeat with the rack ear on the right side.