How do you enter inventory of unused office supplies on an adjustment?

You don't. The amount of adjustment will be for the amount of used office supplies. Normally you are given beginning inventory and a physical inventory amount at the end of the period, so you would take the ending inventory minus beginning inventory minus purchases equals your used supplies.

Your adjustment will be:
Debit Office Supplies Expense for the amount of office supplies used
Credit Office Supplies for the amount of office supplies used