When on the beach, to get the boat off the trailer, simply pull it off. Stand at the rear crossbar, grab the boat at the aft tramp lacings and pull it off the trailer. It will slide off the rollers fairly easily. The only issue may be that the bow will drop off the roller when you get to the end of pulling it off if the trailer is too high. Not much you can do about that unless you unhook the trailer, pull your car away, and then lift the trailer tongue up and pull the trailer out - too much work.
To put the boat back on, stand between the hulls at the front crossbar, holding on to the dolphin striker post. Pull the boat forward on the ground until the bows are at the rear trailer rollers. Lift up on the crossbar so the bows come up and step forward to get the first few feet of the bows on the rollers. Then walk around to the rear crossbar and lift the crossbar up and walk the boat forward on the rollers.
The same thing could more or less be done on a boat ramp, but obviously you will be getting wet. Also, boat ramps tend to be pretty slippery. Personally, I would just back the trailer into the water. It will be much easier. Just be sure to rinse the trailer after doing so and service the bearings every few years.