the little plastic pieces in the middle get cut free.

The servos are put on the tray using the rubber grommets, but not using the little brass eyelets.

The little plastic pieces get screwed onto the tray and act as clamps to hold the servos in place. the sort of triangular ones on the servos that are parallel to each other... the longer straight ones on the servo that is sideways to the other 2 You use the sort of wood-screw appearing screws that have been cut off and "notched" (these are self-tapping sheet-metal screws. They work very well in plastics.)

The servo tray is then mounted in the aircraft on hardwood rails using grommets with the little brass eyelets in place.

There is a place for the switch on the tray... its generally not used.