How do I set up a child support garnishment for a 1099 contractor?

Assuming you have a court order requiring this, I would not do it if you are just asked as a favor.

create a liability account named for the court you make the child support payments to.  Create a service item named for the garnishment and link it to the liability account

on the vendor check

line one = the mapped vendor expense account and the full amount paid

line two = the garnishment item, qty is negative one, and the amount

Later when you pay the court, use the garnishment liability account as the expense (reason) for the check

Would you know if after deducting C/S garnishments on independent contractor checks, would that be a mapped expense I would need to include when running the 1099's?