Doing things backwards - installing Ubuntu after LinuxCNC

Hi Everyone
I have a windows background so Linux and all its derivatives are rather foreign to me. Like many I have moved to LinuxCNC because the cost of windows software was getting prohibitive.

Anyway as well as the CNC router I also have a small desktop laser cutter which I have had to move into the workshop, and that runs with a program called LightBurn. LightBurn requires a 64bit OS to run and there is a Linux based version which I have downloaded, along with Ubuntu 18.04 desktop amd64 which in on a USB drive.

I have done the google thing about setting up my CNC computer as a dual boot,but everyone seems to put Ubuntu on first THEN LinuxCNC.

As a slightly technophobic noobie would someone mind giving me a 'cheat sheet' of instructions on the steps to follow if it can be done.

Thanks in advance.

Hello again tommylight

I need to install Ubuntu as a dual boot on the computer that is already running LinuxCNC. There will be no windows software used in my workshop

The reason I want to install the Ubuntu is because ass far as I can tell LinuxCNC is only a 32bit operating system and LightBurn (lightburnsoftware.com/pages/trial-version-try-before-you-buy) needs to run in a 64bit environment.

My understanding is that with the grub customizer I will be able to select which system starts up, but first I have to somehow install Ubuntu into its own 'partition' of the hard drive. This is the bit that is tripping me up:S