As ocdtrekkie mentions, the CommaAI is an option that offers a little hand holding in the sense that instructions are available. I actually went through the process of building one and hooking it up to my car and deciphering my cars CAN bus messages. It's pretty interesting and would definitely recommend it.
As for making your own, it depends on how good you want it to be. For example, the CommaAI is simple to do even from scratch but not super cutting edge. On the other hand theres companys out there using additional radars and Lidar which increases the cost and complexity. But in theory you'd just need to mount a machine in the trunk of your car, hook it up to your cars computer (have a car capable of being controlled solely via computer), hook up any additional sensors. Then write the software that can fuse all of the incoming data and use it to send messages to the car.
If this is something you're interested in learning about you can check out Udacity's SDC Nanodegree
As for making your own, it depends on how good you want it to be. For example, the CommaAI is simple to do even from scratch but not super cutting edge. On the other hand theres companys out there using additional radars and Lidar which increases the cost and complexity. But in theory you'd just need to mount a machine in the trunk of your car, hook it up to your cars computer (have a car capable of being controlled solely via computer), hook up any additional sensors. Then write the software that can fuse all of the incoming data and use it to send messages to the car.
If this is something you're interested in learning about you can check out Udacity's SDC Nanodegree