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