This is a summary of information in the IQaudIO PDF user manual, with a slight correction as to how to turn off the built-in-audio - kindly provided by IQaudIO support.
1. Edit /boot/config.txt
sudo nano /boot/config.txt
Add these lines somewhere:
dtoverlay=iqaudio-dacplus
dtoverlay=i2s-mmap
Comment out the dtparam=audio=on line:
# Enable audio (loads snd_bcm2835)
dtparam=audio=on
So it becomes:
# Enable audio (loads snd_bcm2835)
#dtparam=audio=on
(Note that this will disable sound via HDMI and headphone jack on the Pi, which is necessary to get Sonic Pi working via the DAC)
Save the file.
2. Reboot
sync
sudo reboot
3. Manually download and run Sonic Pi 2.9
2015-11-21-raspbian-jessie comes with Sonic Pi 2.7, and 2.9 is needed to work with DACs.
I found that sudo apt-get update and sudo apt-get install sonic-pi still left me with 2.7, so I did this:
Download and run Sonic Pi 2.9
wget http://sonic-pi.net/files/releases/v2.9.0/Sonic-Pi-for-RPi-Jessie-v2.9.0.tgz
tar -xvzf Sonic-Pi-for-RPi-Jessie-v2.9.0.tgz
./sonic-pi-v2.9.0/bin/sonic-pi
Sonic Pi 2.9 will then play over the headphones attached to the Pi-DAC+, and it will also control the volume over the headphones.