It will take some stimulation before it becomes completely noticeable, but using fingers, feel inside towards the pubis. Once past the vaginal canal and the pubic bone, you'll feel (dead center) a spongy patch of tissue. This tissue - if you could see it (you'll have to use your imagination here) would look something like the surface of a walnut or a ripply brain. If you push on it, it will usually feel like you need to urinate, or maybe at first it'll feel uncomfortable, like the beginning of that tender/burning sensation if you've ever had a urinary tract infection (very hard to describe, especially since it's different for everyone).
In order to stimulate mine, it works best at first to swipe across it like a windshield-wiper motion, back and forth... while I hear most women work better with a "come hither" top-bottom motion with curled fingertips and more pressure. Once it's "irritated" (stimulated), it'll raise up and become puffy and distinctly noticeable - THIS is your g-spot. Once you've gotten that far, it's trial and error as to what feels best for your particular body.
On a personal level, I need at least 30+ min of either foreplay or other type of play before my g-spot is even ready to take enough attention to cause a reaction, but that's just me. And when it finally occurs, I need to either be fully relaxed or lightly bearing down for a g-spot orgasm to be produced, where I tighten up for a clitoral orgasm.
Like most, mine feels (at the beginning of the "orgasm process") like I have to urinate, but I have figured out how to clear my mind of the thought (or just not care because of towels, etc.) and once I can get past the thought of peeing, a warm sensation spreads from the g-spot area outward... once it feels like the warmth has passed into my legs, I almost always orgasm riiiiight then.
Again, different for everyone - but that's my "procedure" in my own experience of both finding and stimulating the g-spot. I've used the process on a few others with great success, so at least in my case, I must be doing SOMETHING right that works for more than just me!