How to Launch a Boat
To launch a boat, start by unplugging the trailer towing light assembly and putting the drain plug back in the boat. Then, undo the hold down straps while leaving the winch hook in the bow eye. Next, slowly back the trailer down the launch ramp until the water is just above the hubs on the trailer. Once you've backed the trailer down the ramp, get out, crank the boat engine, and undo the winch hook from the bow eye. Finally, back the boat away from the dock.
Prepare the boat. Prepare as much as possible while the boat is on the trailer, and on dry land. Prepare the engine, fuel, safety gear, mooring lines, fenders, etc. You should be ready to drive the boat off the trailer right after it's backed down the ramp. Other people want to launch as well, and tempers will flare if you use dock time to do things you could have done in the parking lot. It is a good idea to have your own written checklist. If you're launching a small boat into the water, make sure to put the drain plug in so you don't capsize the boat. Make a plan together. If bringing along guests, brief them on where to go and what tasks to do. Be sure to let the guests in the boat first, along with the boat operator. That way nobody has to jump in after. If bringing along children, forbid young children and pets from playing or loitering on a boat launch ramp as this is a roadway that transitions into a waterway. Keep children and pets supervised by an adult in a different area than at the boat launch ramp. NEVER let children or pets play or be unattended at a boat launch ramp. Unplug the trailer towing light assembly. Unplug the trailer towing light assembly from the vehicle's electrical system. Put the drain plug in the boat. This is very important! Undo the hold down straps. Undo and stow the hold down straps, but leave the winch hook in the bow eye. Have your dock lines and fenders ready to go for immediate use.  Back the trailer down the ramp. This is easiest done with two people: one driving and one as a spotter. With a truck or SUV it can be easier to see when backing up if you put the tailgate down or open the rear hatch/door/window. Drive slowly, making tiny fine-tune steering corrections as you go. How far to back the boat trailer into the water depends on many factors - type of trailer, depth of the water, type of boat, etc. A good rule of thumb is to back in until the water is just above the hubs on the trailer. Be careful about backing in too far otherwise the tow vehicle might become stuck. When backing down the ramp, have the vehicle in neutral. This makes it easier to control the vehicle quickly. If something fails, quickly put it in gear and go forward. Ensure that your tow vehicle has the parking brake set before exiting the vehicle. When getting out of the vehicle on the ramp, apply the handbrake first, check that it is holding fast, and then put it in park. When the vehicle is sitting in park, the whole weight is sitting on a little piece of metal in the trans (parking pawl). Should this break and the park brake doesn't hold, you launch the whole rig. Lower the lower unit (for boats with outboard and inboard-outboard engines).  Turn on the bilge blower to remove any gasoline fumes that may have accumulated in the bilge.  Ensure that no person is near the engine when starting and that no person or boat is in your path before you put the boat in reverse.  Pay attention to currents, wind and waves that can easily pull your boat off course and into a collision course with another boat or dock.  Crank the engine. Make sure the engine is in the water, and turn it over. Once it's running, undo the winch hook from the bow eye, and back the boat out of the trailer. For smaller boats, you can just push the boat out while holding a mooring line. Secure the boat to the dock. Tie the mooring lines to cleats on the dock. Use fenders to prevent the hull from being scratched. If you forget to tie a dock line to the boat, you won't be able to sure your boat after you've launched it. Slowly pull the vehicle out. Boat ramps can be slippery with moss and algae and it is important to not let the vehicle's tires spin. If your tow vehicle is equipped with four-wheel drive, this can be useful if your back tires start spinning. Once the boat is afloat and secured to the dock, drive the vehicle back up the ramp and park it at the landing, in the designated parking area. When pulling away from the dock, it is important to understand that unlike a car, the stern of the boat is pushed to one side or the other, whereas a car's pivot point is in the front. Do not turn the boats steering wheel in the opposite direction of the dock, with the boat a mere few inches from the dock, and just hit the throttle - or your engine's outdrive will push your boat right into the dock, scratching or gouging the gelcoat in the side of the hull. Make sure that you have pushed away several feet from the boat dock before driving away, so that your stern has ample room to turn the boat without scraping the boat alongside of the dock. 
Prepare the boat. Make a plan together. Unplug the trailer towing light assembly. Put the drain plug in the boat. Undo the hold down straps. Have your dock lines and fenders ready to go for immediate use. Back the trailer down the ramp. Ensure that your tow vehicle has the parking brake set before exiting the vehicle. Lower the lower unit (for boats with outboard and inboard-outboard engines). Turn on the bilge blower to remove any gasoline fumes that may have accumulated in the bilge. Ensure that no person is near the engine when starting and that no person or boat is in your path before you put the boat in reverse. Pay attention to currents, wind and waves that can easily pull your boat off course and into a collision course with another boat or dock. Crank the engine. Secure the boat to the dock. Slowly pull the vehicle out. When pulling away from the dock, it is important to understand that unlike a car, the stern of the boat is pushed to one side or the other, whereas a car's pivot point is in the front. 