There are two common ways to put a newbuid to water. The method which is used by most shipyards today is assembling the ship in drydock and when it's ready to be put into the water, the drydock is slowly flooded, which raises the vessel off of the keel blocks it had been built on. When water level inside the dry dock meets water level outside, the large gate which holds back the water is removed and the newbuild is floated out.
The older method is outdated and more fascinating. The ship is built on large wooden ramps that lead to the water. It is more difficult to build it on angle than in a level dry dock, but still practiced in a number of shipyards. When the vessel is ready to be sent to water, ramp's lower part is flooded and the gate removed.