you start Processing IDE ( now 3.5.3 )
Tools / Add Tool… /
(TAB) Libraries / Arduino ( Firmata )
now rev 9
https://playground.arduino.cc/Interfacing/Processing/
minimal starting code
import processing.serial.*;
import cc.arduino.*;
Arduino arduino;
void setup() {
println(Arduino.list());
arduino = new Arduino(this, Arduino.list()[0], 57600);
}
make sure on the port ( list[0] ) a arduino is connected
were you have loaded the Firmata code!
if you want work with your own arduino code ( not firmata (protocol ))
pls see
https://processing.org/reference/libraries/serial/index.html