First up, make sure everything is in good working order. This includes checking your trailer hitch and safety chains are firmly secured. Before dipping the rear end of your trailer in, you're advised to disconnect the power from the tow vehicle to trailer so that you don't short your brake lights.
Give the boat ramp a good once over to check for any submerged objects, slippery spots or, for those in the north, lurking crocs.
Prepare for Launch
Once your trailer is in position, just before you head down the ramp, attach your winch hook to the bow eye and undo the tie-down straps and pack them away. Trim your outboard so that it doesn't drag on the ramp and make sure drain plugs and bungs are in tight. If you're going to need dock lines and fenders to tie up to a dock once your boat is in the water, then have them in place and ready to.
Back It Up
Once you're satisfied that everything's good to go, have someone position themselves somewhere you can see them and they can see the back of your boat – it's much easier if you have someone to guide you in this way. Some boaties with suitable 4WD tow-vehicles will reverse in low range, while others will keep the car in neutral so they can quickly shift into gear and drive forward if something goes wrong.
Make small adjustments to the steering, take it slow and don't be afraid to drive out and reassess if things don't feel right.
Know When To Stop
Stop reversing when the stern of the boat begins to rise from the trailer bed. At this point your helper can get in the boat and detach the winch.
Out You Get
Pull your handbrake and put your vehicle in park. Make sure its well and truly stopped in place before getting out to help push the boat off. Larger boats may require you to lower the engine and reverse off the trailer.
Have your helper toss you the dock line so you can secure the boat before heading off to park the car.
Once you're ready to call it a day, shut off and trim the engine then fetch the car and and back the trailer down the boat ramp. Attach the winch to the bow and line the boat up with the trailer, being mindful of the possibility that the stern may drift a little. Crank the winch to pull the boat up onto the trailer and apply the brake once it's in place. You can take care of the tie down straps and reconnecting the lights once you've pulled away from the ramp, just be sure everyone's out of the boat before you start driving out.