• download the latest serial amd64 installer for OPNSense (e.g. OPNsense-17.7-OpenSSL-serial-amd64.img.bz2) 
• Write image to an USB stick using Etcher (Etcher is a graphical writing tool for Mac OS, Linux and Windows)
• boot apu2 from USB with installed mSATA/SD
• follow the instructions of the installer

Important: the SD slot on the apu2 boards is directly connected to the CPU and needs the module SDHCI to work.
Do not use an SD to boot FreeBSD based OS like pfSense and OPNSense on apu2 boards. Only very recent kernel versions fully support booting via SDHCI.
Try an USB-SD adapter in case of related troubles. If it works with the USB-SD adapter, but not in the SD slot, then the SDHCI module is missing ...

Installation Steps¶

The installation process involves a few simple steps.

Note
To invoke the installer login with user installer and password opnsense

Tip
The installer can also be started from the network using ssh, default ip address is 192.168.1.1

1. Configure console - The default configuration should be fine for most occasions.
2. Select task - The Quick/Easy Install option should be fine for most occasions. For installations on embedded systems or systems with minimal diskspace choose Custom Installation and do not create a swap slice. Continue with default settings.
3. Are you SURE? - When proceeding OPNsense will be installed on the first hard disk in the system.
4. Reboot - The system is now installed and needs to be rebooted to continue with configuration.

• Not Booting from the USB Stick