Putting it together

Build it in stages and check each stage before the next. The full pin tables are on the wiring page: keep it open while you work.

Step by step

Flash the memory card. You can do this first, while the iron heats up: see flashing the firmware.
Prepare the Pi. Solder a 40-pin header to the Pi Zero W, or plan to solder wires straight to the holes you need.
Fit the front panel. Mount the OLED and the eight LEDs in the case front, in the order PWR, AA, OH, CD, RD, SD, READY, NET from the left.
Wire the power. Barrel socket to the Pi's PP1 (5 V) and PP6 (ground) test pads, found by their printed labels on the board, and to the amplifier's Vin and GND. Before soldering, check with a meter that PP1 has continuity to header pin 2 or 4 and PP6 to pin 6. Run the 3.3 V and ground from pins 1 and 6 to the modules. See the safety notes on the wiring page: this wiring has no fuse.
Wire the I2C bus. SDA (pin 3) and SCL (pin 5) to both the OLED and the MCP23017. Set the expander's address to 0x27 (see wiring).
Wire the lamps. From each GPA output through a 470 ohm resistor to the LED's longer leg; the shorter leg to ground.
Wire the amplifier. BCLK to pin 12, LRC to pin 35, DIN to pin 40, and the speaker between OUT+ and OUT-.
Wire the DA15 for USB. Pin 10 to the Pi's USB D- pad (PP23), pin 11 to D+ (PP22), pin 5 to ground. No 5 V goes to the connector.
Make the PC cable. DA15 plug: pins 10, 11 and 5 to the USB-A plug's D-, D+ and ground; the USB +5 V wire stays unconnected and insulated. Pins 1-9 go to the same pins of a DE9 socket for the serial side.
The serial port (optional, being tested): three MAX3232 boards and five pull-ups, wired to DA15 pins 1-9 as in the wiring table.

Before the first power-up

Check with a meter first

With nothing plugged in, check that there is no short between 5 V and ground, and none between 3.3 V and ground. Check that nothing at 3.3 V touches 5 V. Check PP1 to header pin 2 or 4, PP6 to pin 6, and the barrel socket's contacts with the plug in. Look over every joint for stray strands of wire.

Keep the PC unplugged for the first start, and never connect the PC's USB while the box is on barrel power unless the cable's +5 V wire is cut.

Put the flashed card in the Pi.
Plug in the 5 V supply: a current-limited bench supply for this first start if you have one. Don't touch or move bare wiring while it is powered.
Wait for the screen. The first start takes longer than later ones, while the box makes its own passwords and certificate. Then the screen shows SETUP: JOIN WIFI with the name and the password of the box's setup hotspot, and the PWR lamp is lit. (PWR means the box's lamp service is running, not just that power is there.)

If the screen stays dark or the lamps stay off, unplug the supply and see troubleshooting.

Check the lamps, screen and sound

Once you have done the first-time setup, open Maintenance > Diagnostics on the setup page. It can test each lamp, the screen and each sound, and play a full demo call. Check each lamp lights in the right place. Start the sound tests with the volume low and turn it up from there.

Next: flashing the firmware.