Powering motors and the arduino in a car

What would I need to do to take the 12V power supply from the car and convert it to 5v for the arduino and run 12v to the motors?

Would I also need some way to try and stabilise the current/voltage to not fry things?

A switch mode buck regulator to drop the 12V to 5V for the Arduino. Pololu has a range of step down converters. The converter will take care of most of the problematic motor noise. Caps across the motors can reduce electrical noise from motor brushes, too.

Which Arduino?

Just an uno. Would also be stepper motors so noise not much of an issue. so would i be ok to directly wire from a 12v connection int he car to my motor drivers?

Draw and post a schematic, please. That way less chance of mistaken interpretation.

Well its not that complicated really I just have some a4988 and an arduino uno