Cut the prototyping board to the size of pi zero
Solder the 2x20 Female Pi header onto the prototyping board
Solder the header pins onto the I2S DAC and audio amplifier boards and mount them on the prototyping board.
Wire the boards as indicated in the wiring diagram
Wire the Left, Ground and Right pins of the I2S DAC to the Left, Ground and Right pins of the audio amplifier.
Use the one-line installer to setup the DIY DAC.
curl -sS https://get.pimoroni.com/phatdac | bash