Since the schematic is very simple, I used a prototype board with wires direct soldered on the components. Only one note: the internal voltage regulator got very hot if used to feed the power to the DAC. So I used an external classic LM7805. And the only advantage of the Digispark board over a single ATtiny85 chip has gone...

You need the Arduino IDE configured to support the programming of the ATtiny85 chip, and the chip as to be programmed with an ArduinoISP. Follow this instruction:
Programming your Attiny85 with an Arduino Uno as ISP
Remember to add the two libraries included in the files section.

If you have a Digispark board with bootloader, since my code is written for a bare ATtiny85 chip, you need to upload via usb from command line. Enable verbose output in arduino IDE, copy the path of the hex file generated from the log, and issue this command:

micronucleus /tmp/arduino_build_871172/IRvolume85.ino.hex

(replace with correct path)