The Easy Way
Using this wiring kit, you really just need to plug in the motors, tape or otherwise secure the connection and go.
The other way
The stepper for the z axis just needs its wires extended. Usually you would just cut the plug and splice in some 4/24 shielded cable and be done with it.
Same amp draw as a single stepper, ideal.
Hand drawn diagram
Modified parallel harness
Twice the amp draw as parallel.
The steppers for the X and Y axis are wired in parallel, with the far motor getting one coil wired in reverse.
In this picture the yellow and blue are a pair/coil, and the green and red are a pair/coil. So to reverse this stepper you would wire this blue to the other yellow, and this yellow to the other blue. Do not get caught up in my colors, the position on the plug tells you what 2 wires are pairs.
To wire in parallel you connect all the like colors and plug it into the ramps board, then on the far stepper change on pair/coil.