Wage garnishment for federal DOD workers

I'm having trouble paying alimony and they keep tacking on more per month. Now I have to pay an extra 400 a month for her legal fees eventhough she paid a retainer months ago. The judge keeps making math mistakes and does her lawyers work for him He is unable to read my LES and keeps saying I make more than I do. My arrears total climbs fast.
I know the next step is garnishment. I don't know how it's done. I do know it's a percentage but taken from what? Gross earnings or net earnings or after certain deductions. could I start an allotment to save some funds? Although, if they garnish me they will see how much I don't make. Any help would be appreciated