To begin, craft an EU Injector and a Forcefield Core. Place the EU Injector, then the Forcefield Core directly on top of it. The Forcefield Core does not work unless it is on top of the Injector, as opposed to beside or below it. Next, wire the EU Injector up to a source of power, and give them both a redstone signal. This can be done by either placing a lever on either block and switching it on or using another redstone method. When the Forcefield Core and EU Injector are activated via a redstone signal, it will begin to draw Force power from the EU injector which in turn will draw EU from the cable it is connected to.
There is no known limit to the voltage the EU Injector can receive. It has been tested with High Voltage (512EU/t) Directly from a MFS Unit sent through Glass Fibre Cable without exploding. Extreme voltages (up to 2046 EU/t) have now been tested and work as well - 2046 EU/t rather than 2048 EU/t due to power loss in transmission.