Debian 9 post installation steps - Hortensie/CNC_x_axis_linuxcnc_mesa_5i25_7i76 GitHub Wiki
cd /etc/default
sudo nano grub
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash isolcpus=1,2,3"
sudo update-grub
sudo reboot
cat/proc/$$/status/
Open the Synaptic Package Manager
search for linux-image and install linux-image-4.9.0.6-rt (Version depends when you check this guide)
reboot the pc
in a terminal type uname -a it should report back PREEMT RT
Linux d9-uspace 4.9.0-6-rt-amd64 #1 SMP PREEMPT RT Debian 4.9.82-1+deb9u3 (2018-03-02) x86_64 GNU/Linux