Using Arduino to send an RS485 Command

I am pretty new to Arduino. But I would like to use this device to send an RS485 command to a device when I push a momentary button. I want to push the button to send an "ON" Command to the device and then push the button again to send the "OFF" Command. Sounds simple in theory but I not sure what all pieces are needed to put this together.

You could do worse than start here with Nick Gammon's work.

He provides a library to do the heavy lifting.

His example there assembles a message from master to slave, containing a slave id, a command (turn led on) and value of brightness based on reading a pot.