New to linux and a bit lost

I have an old computer and installed Ubuntu on it that went fine. I am trying to install the Linuxcnc with no luck. I tried the flashdrive and can boot it from there but install tutorials show people doing 2 things, 1 the hard drive with cnc or a cd drive. the cd drive I try and right click the ISO but do not have the option for write to disk.

This computer will be dedicated only to my cnc. How do I just use it only for that and not have to select a boot drive everytime? Do I install just linuxcnc or do I need Ubuntu?

What Tommy says, then after you have booted, click of the desktop icon to install a new OS preinstalled with Linuxcnc.
You can choose whether to replace Ubuntu (easy) or keep Ubuntu and dual boot (a little harder).

It would help if you are able to share the specs of your computer.

Ok I did some searching and linuxcnc is not an option right now as I need different controller. What I have is gbrl board so will need that software. I downloaded candle gbrl for linux i86. Installed Linux Mint and trying to install candle. Thats where I am lost now. I am new to linux as far as I just learned to install it. In the folder for candle is nothing I can see to install it, How do I install programs in linux? I will just need to search how I know this forum is linuxcnc not candle