I would not say that the setup guides from 3CX are vague - it is really that simple to setup the 3CX SBC. From the 3CX side they cover everything they should from the point of having your OS installed.
That being the case installing the Windows SBC (see guide from Nick above) should be a breeze, however if you want to use the Raspberry Pi or Linux variants you would need to:
Linux: Acquire the OS
https://cdimage.debian.org/debian-cd/current/amd64/iso-cd/ and using a tool such as rufus make a boot-stick and load it onto your machine of choice. You can then follow the graphical installation to get your system up and running.
Once logged in a "root" then run an apt-get update and follow the guide:
https://www.3cx.com/docs/3cx-tunnel-session-border-controller/
Raspberry Pi: Just buy the "Pi Kit" with OS pre-loaded (much easier), and follow the above guide also.
I do also know of some 3CX partners who offer pre-loaded Raspberry Pi's which they will configure with your requested settings and ship to site. Then only plug and play provisioning over SBC is required (for 3CX Supported/Preferred Phone models only - Yealink, snom, Htek, Fanvil).
The advantage of the Windows SBC is that it is easy to setup and configure (as Windows knowledge is most common) but you will need to buy a Windows licence for the environment obviously.
Linux is Open Source (so no licence is required to be purchased) however you will need a little Linux knowledge to get the SBC up and running.