To add a Garnish to a Ledger, go add, Prepaid, Garnish Order.

First choose the Ledger that we must deduct money from when the customer pays money into it. Second, choose the Ledger that we must transfer a portion of the money to.

Specify the maximum percentage of the payment that can be deducted. Normally, the second Ledger would be negative, representing the debt owed by the payer. So when that amount reaches 0 (zero), the Garnish Order will stop deducting money from payments.

If you want the Garnish Order to always deduct the money, not only if the second (claimant) Ledger is negative, there is a checkbox on the add screen you can use to specify that.