First, construct a switch for your Raspberry Pi GPIO pins. I generally use MOSFETs for these low-voltage operations, with the gate to a GPIO pin.
Some sprinkler systems have a moisture / rain sensor that open or close a circuit to disable or enable the next scheduled irrigation. Usually if a sensor is not installed, there's a jumper wire to keep the circuit open. Replace the jumper or sensor wires with the open & ground wires on the relay or MOSTFET wired to the Pi. --> DO NOT WIRE THE GPIO PIN TO YOUR SPRINKLER SYSTEM <--. You'll likely frizzle-fry the whole thing. You want to close the circuit, not send any amperage to it. Seriously. Don't move on unless you understand your irrigation system. SERIOUSLY.
If necessary, enable the sensor or disable the sensor bypass on the irrigation controller.
Wire everything to the Raspberry Pi and install the SprinklerSwitch software as instructed at https://github.com/deckerego/SprinklerSwitch