My ex-wife owe me over 4k in back child support and she recently just starting paying her child support. I would like to know how do I put a lien on her taxes.

You should speak with the court clerk in your county. Generally this is either handled through them or through a state office. Either way, the court clerk in your county should be able to get you to the proper person to help.

If the court knows about the back child support, I believe that the lien on the tax refund will be done automatically

I've moved this to the Family Law forum since the issue is Child support more than taxes.

Most states have an agency to help collect child support. So you should be working through that agency, or the Family court that issued the order.