Almost the same as with Armbian - you get the RT kernel patch and apply it to the kernel you build (if it applies at all). At some point, the RP kernel repo had a RT branch [1], maintained by a 3rd party developer, but hasn't been updated for quite some time. AFAIK the default Raspi OS supplied kernel doesn't have any RT patches added, but you can ask in the RPI forums.
[1]https://github.com/raspberrypi/linux/tree/rpi-4.14.y-rt