Solder 12 straight male headers onto both the left and right sides of the Arduino. The long ends should be on the underside of the board.
Solder 6 right-angle male headers onto the side of the Arduino opposite the Reset button. The long ends should be on the underside of the board.
Solder the straight headers onto the side of the protoboard with the square pads. From the square pad side of the protoboard with the logo in the top right corner, they should be soldered as far right as possible and 7 pads from the bottom.
Solder the transceiver (SN75176B) anywhere on the board, but it makes the most sense to connect it close to the protoboard's logo. Connect pins 1, 2, and 5 to ground from the Arduino, and connect pins 3 and 8 to Vcc from the Arduino. Connect pin 4 of the transceiver to pin 10 on the Arduino.
Using the solder cups in the XLR jack, solder wires to pins 1, 2, and 3 on the XLR hack. For extra safety, also solder the chassis to pin 1.
Solder the lead from pin 1 on the XLR jack to ground from the Arduino. Solder the lead from pin 2 on the XLR jack to pin 7 on the transceiver. Solder the lead from pin 3 on the XLR jack to pin 6 on the transceiver.
Solder one lead from the resistor to pin 9 on the Arduino. Solder the other lead to an empty pad.
Cut the male ends from the jumper cables and strip the wire back so you are left with just the female ends with some leads. Solder one of the leads to the free lead from the resistor, and solder the remaining one to ground.
Connect the LED to the female headers: the short lead goes on the grounded header, and the long lead goes on the resistor header.
Connect the FTDI breakout board to the right-angle male headers on the Arduino.