All I have to do is stimulate my clitoris. I don't need penetrative sex or masturbation to squirt, but I can squirt from it.
I just get a basic vibrator, set it on it's highest, place it on my clit and within 5-10 seconds I can squirt. My bf is still amazed at how quickly I can do it.
My problem used to be that it was preceded by an intense orgasm and to be honest, I don't like intense orgasms so I would always prevent myself from having one so I didn't squirt for a while.