You can't.
RS485 is a differential, open-collector bus.
There are two signal wires, D+ and D-. A logical "1" is encoded when D+ is more positive than D-, and a logical "0" is encoded when D+ is more negative than D-. These two voltages are positive and negative, relative to a "common" voltage.
Additionally, when a device is not communicating via RS-485, it MUST tri-state both D+ and D-. You =can= tri-state a digital pin with an Arduino, by making it an "input" pin, but you cannot drive the RS-485 wires with the 0 and 5 volt signals.
The solution is to add a chip such as the Maxim MAX485. It's a =very= simple device and includes it's own circuitry to produce the required voltages.