The aim of fluid loading is two-fold:
1. to get you used to letting your bladder fill up to a point where there is a comfortable and strong urge to pee. Do not let it go beyond that where it gets uncomfortable and you are bursting.
2. By doing a three second pee, you can return after a couple of minutes to have another 3 second pee; you can continue like this for as long as you need for desensing. This way your desensing session of say an hour can contain easily a dozen pees, and so you met better progress. You can sip water to keep your level of urgency at the right level. We tell guys to drink a litre of water about an hour before they want to start desensing; obviously this can vary on individuals.
Does that make sense?