Same as any other user defined deduction like Health Insurance or 401K.  Make sure you define it in the division as a $0 amount and then assign it to the appropriate employee with an amount.

We set all garnishments up to a clearing account GL code, which we verify at minimum monthly is at zero.  
 
We then create an AP vendor for the Garnishee recipient.  We charge the clearing account for the exact amount taken from the employee as an AP voucher each pay period when the employees paycheck is garnished.
 
This can be set up as a recurring AP voucher for the exact amount and period of the garnishment.  We run recurring AP vouchers at the same time that we do payroll and make sure the the clearing account is zero before posting either in GL-O.
 
The biggest problem with garnishments is making sure that all of this balances.
 
Jeanette Zimmerman
Lowery & Company Inc.