How do I assign a lien on a car?

Unless you are a mechanic who worked on the car and were not paid for that work, you can't. Even then it would depend on where you are as to whether the law allows it and what is required to record the lien.

More details may help us help you.

They can bring the title in and sign papers showing they borrowed money and you can be put on the title as a lien holder.

You can ( in some states) sue for a judgement and get a lien on their car, ** but if they already owe on the car it does not really help you much.

From your other question, it appears you have someone, or a group of people that owe you money due to a school you run. Perhaps if you post all of the details of the type of school and the issues you have we can help better.