The top part of the COMMUNICATIONS ANTENNAS represents the low-gain antenna. The remainder of the structure is the meduim-gain antenna.
Cut out the COMMUNICATIONS ANTENNA HALVES from Sheet 2. Slit the central lines, and fit them together, slot into slot, at right angles.
The two arm-like objects on one piece represent the thrusters mounted on the +Z side of the spacecraft. They are part of the propulsion subsystem, not the communications antennas.
Apply glue to the base of the ANTENNAS assembly.
On the top of the SPACECRAFT BUS, apply glue along the central black line which has arrows at its ends. Set the ANTENNAS assembly down onto the top of the SPACECRAFT BUS, rotated so the thrusters are in line with the arrows. Straighten it up, and add glue where needed.