The easiest way is to download and install the Debian Stretch test ISO:
www.linuxcnc.org/testing-stretch-rtpreem...-uspace-amd64-r8.iso
This has the advantage that it contains the required uspace version of LinuxCNC, plus a recent Preempt-RT kernel
so is an especially good choice if you have newer PC hardware. It has the disadvantage that there may be some problems with some LinuCNC GUIs that depend on Glade (gmoccapy is one example)
Another way is to start with a Debian Wheezy distribution and install the Preempt-RT kernel and LinuxCNC uspace package:
linuxcnc.org/docs/html/getting-started/g...th_preempt_rt_kernel
This is a better choice for older hardware, and should not have any GUI issues since is the same distribution
the current released version of LinuxCNC uses.
Yet another option is to install LinuxCNC from scratch on a Linux distribution like Ubuntu or Mint, here is a very long thread on this:
forum.linuxcnc.org/9-installing-linuxcnc...-or-deb?limitstart=0