Starting from a fresh Debian 12 install without linuxcnc-uspace installed

Download the deb files from here
https://onelaboratories.us/rtai_lcnc/ 
copy each and install using: 

from the dirctory of the deb files:
sudo dpkg -i (place the deb file here)

reboot and choose the RTAI kernel

IF linuxcnc-uspace is already installed then you need to run:
sudo apt purge linuxcnc*

Grub Customizer set: quiet isolcpus=1 (2,3) idle=poll
sudo update-grub