TinkerGnome helped me set up the Marlin build environment. How to do it is out there and published but I wrote down extra details. I don't know if my notes make sense but I thought I would post them here. If you try to repeat this and have any trouble let me know below and I will clarify the instructions as necessary.
Get the source code from github and put somewhere on your hard drive. I'm not going to give you a tutorial on github - there's a lot to github features alone - you can simply