You don't really need to ARM the ESC's, but you may need to calibrate them one time before first use. To calibrate them -
You need to send them a a repeating waveform that consists of 2 milliseconds HIGH and 20 milliseconds LOW (the low period can vary a bit, but the HIGH period can not) WHILE you apply power. Then, when you hear a series of tones from the motor the ESC is connected to, you change the waveform to 1 millisecond HIGH and 20 milliseconds LOW. When you hear another tone, remove power.
Then they are calibrated. The calibration procedure sets the maximum and minimum pulse widths you will later use to control the motor.
You can also use a "servo tester" that you can buy on Ebay for this procedure. Make certain you get the one with a digital readout so that you know what pulse width it is putting out.
You only have to calibrate one time. After that, you can control the motor with pulse widths between 1 and 2 milliseconds high, 20 milliseconds low. You should also know that the ESCs will not start the motors if they get a pulse width of greater than 1 mSec high while power is initially applied to the ESCs. In other words, you cannot start a motor at "full throttle", you can only start at minimum throttle. This is for safety.
The term "ARM" comes from flight controllers, not ESCs. A Flight Controller must be ARMed before it will allow the motors to spin. But the flight controllers control the ESC. So the term ARM does not apply to ESCs.