While swinging, I have had the opportunity to practice this, but there is a mental aspect as well as physical. A woman can squirt without orgasaming, they are not the same thing. A woman will squirt when her skene glands are stimulated. There are two of them, one on either side of her urethra. The fluid comes out of her urethra, but it is not pee. If she did not empty her bladder beforehand, she can release urine while squirting. It is near the G-spot, but not the same thing. All woman have the physical ability to squirt, there is usually a mental block to the sensation of letting go.
First, she needs to be very turned on, ideally after she has an orgasm, but she is wanting more.
Second, she needs to trust you. She is going to have to fight through the feeling of wanting to pee. Have her empty her bladder before you start, that way she won't worry. I have found that letting a woman know that you find it super sexy and how turned on you are usually lets her relax that extra-little bit.
Third, insert two fingers palm up while she is lying on her back. Make a "come hither" motion, pushing the front wall of her vagina. You should feel a rough patch. Go slow and massage that spot and the area in front of that. You can also alternate between that and a windshield washer motion.
MAKE SURE YOUR NAILS ARE CLEAN AND CUT. You are playing in the general area of her urethra, you can easily give her a UTI from this.
As you stimulate her, she is going to feel the need to pee. Tell her to push as though she is trying to pee. Her bladder should be empty, so she won't have anything to let out. After a while, you should feel the area behind the rough patch fill-up with her ejaculate and get hard. You are pushing the urethra closed, so nothing will come out until you remove your fingers.
The same woman may like it super-rough one time and super slow the next. Just communicate with her.
I have also noticed that once you do this, it is easier the next time with the same woman. You are also teaching her, if she has never squirted, a new sensation to concentrate on. Most women can squirt during sex after they have been stimulated like this.
Do not approach this with a "mission" attitude. Just have fun and watch her squirm. If it happens, great, if not, try again. She may have to do it to herself at first with a G-spot vibrator.
Hope this helps.