I'll answer for my girl since we found out together that she was capable of squirting. It took slapping her clit with my fingers for her to squirt the first time and is the easiest way for her to do so now. Other methods are slapping her clit with my cock (my fave), diddling her clit with fingers using a fast side to side movement or g-spot massaging but pulling out fingers or other object to allow the squirt to project outward.
Any girl is capable of squirting, but she must be willing to let go and not be concerned about having a sensation that she will urinate. She must just let "it" happen and not be uptight.