This instructable talks about driving multiple bipolar stepper motors (such as used in RepRap 3D printers) from a single driver, such as a Pololu Shield. Each motor is driven via two circuits, call them 1 and 2. Each circuit has two wires, call them A and B. The wires are connected in order 1A, 1B, 2A, 2B.
To connect multiple motors to a single driver, you can wire them either in parallel, or in series. Either will work, but motors wired in series will produce more power. To see why, consider the electrical characteristics of the motors and the driver.
RepRap motors are often rated 2A and 1.1ohms. Using Ohm's Law, such a motor needs 2.2V of power. Pololu Shield is rated 2A and gets 12V from the RAMPS board (or at least I think it does). If you wire the motors in parallel, every motor gets the full voltage, but the current is split, meaning that the total current is still only 2A - no matter how many motors you add, there is no more power. I found this out the hard way, when I was building a printer where I used four motors to lift a fairly substantial gantry in the vertical direction. It would not budge.
In contrast, wiring the motors in series splits the voltage, but every motor gets the full current. Given how little voltage each motor really needs, you should be able to get five times the power if you use five motors. In my case, the same four motors wired in series had no problem lifting the same gantry with power to spare.
To wire the motors in series, split the 1A and 2A wires from 1B and 2B wires in the cable coming from the driver and connect to the first and third pin on the header where you'll plug in the first motor. Connect 1B from the first header to the 1A on the second header (i.e., the header where you'll plug in the second motor). Connect 2B from the first header to 2A on the second header. Continue this A to B connection as you add more headers. On the last header, connect 1B and 2B pins to the 1B and 2B wires going to the driver. See the wiring diagram.
See the picture of my distribution board with two Y motors and four Z motors, each set wired in series. To map the picture to the diagram, left is top, two wires are connected on the left side and two on the right.
Keep in mind that, to have the motors work, you must have all the motors connected, otherwise the circuit is interrupted. If you for any reason don't want to fully populate your headers, use jumpers to short 1A to 1B and 2A to 2B where you have any motors missing.