If you serve 30 days or more on active duty and then separate permanently with a DD214, you can deduct your military pay from your West Virginia taxable income for the year you were discharged.
There is nothing to file in advance. Claim the deduction on the West Virginia personal income tax return you file for the year of your discharge, and keep your DD214 with your records.
Your DD214 showing your separation date, and your Leave and Earnings Statements or W-2 for the military pay you are deducting.