I think absolute zero could possibly be confusing terminology. Your absolute coordinate would be machine coordinates, which typically never change. When you zero to a part, you change the work coordinates. To do this, you simply jog to you're zero position, and set the axis coordinates to zero with onscreen buttons.For example, how do you position a piece of wood and tell the machine where absolute zero is on the piece to be cut relative to the GCode generated by the software?
I don't know if there's a step by step tutorial anywhere, but a good start might be to watch the tutorial videos at ArtSoft USA - Home of Mach3 and LazyCam.
Feel free to ask any questions you may have.