You can start by downloading MachScreen. It is a very easy to use screen building program for Mach3.
Is free to use and will not take you long to learn to use it.
There is a thread here on the forum for just MachScreen that has a lot of additional information.
I built SimpleMill using it. Download SimpleMill here:
http://www.mrrace.com/SimpleMill/ It is a simple 3 and 4 axis screen set.
Start by using MachScreen to play with SimpleMill screens, that way you will not be messing up your current Mach3 screen set.
Good luck with your project and please keep us up to date as you start putting it together.
George