Starting a car without a key requires bypassing the ignition lock. This can be done by connecting the battery cord to the ignition cord, then touching the starter cord to the connection. This process is simplest on pre-1990s vehicles. Post-1990s vehicles come with immobilizers and alarms that must be bypassed. Many modern cars offer a keyless start button.
Before a car can be started without a key, the person engaging in the process must know what the battery, ignition and starter cords look like. These can look different from model to model, so learning what they look like requires research.
Once the cords in question have been identified, three clip leads and a screwdriver are necessary. The panel under the steering column should be opened to provide access to the wires. The insulation around each wire should be removed, being careful to avoid touching them to metal or each other. The leads should be clipped onto the exposed portion.
Once that is done, the only thing left to do is to connect the battery to the ignition cord, which should cause the car lights to come on. At that point, the starter should be touched to the battery and ignition cord combination. It should not be clipped on, merely touched to the others, and care should be taken to avoid sparks.
Once that is done, the car starts. It is not possible to drive the car unless it was made before 1969. Cars after 1969 were made with a steering lock that must be activated by a physical ignition key or by the electronic control unit.