How to convert a cars alternator to a motor?

This project i not recomend. Better buy new motor if you need motor.

Alternator is one name of generator
Before that need to know what type of this alternator, is that permanent magnet DC generator (dynamo), induction generator or sycronous generator.

From basic concept, it can act as genarator and motor. if voltage supply to it, it will become motor. If rotation give at the it rotor, it will become generator.

Normally generator produce AC voltage. For alternator, there have electronic component to convert this AC voltage to DC voltage. This converter call as rectifier.

Since you need to convert alternator to motor. You need to remove this rectifier circuit. then you can supply AC voltage but you need to design the AC voltage spec as follow the alternator winding spec. I dont think you can find this spec. You need to find the frequency, voltage, current spec for this winding. You can try to use external variable AC supply.

gud luck & take caution of danger.