How do we prepare adjusting entries for, Supplies on hand at the beginning of the accounting period are $310. Supplies on hand at the end of the period are $190

The difference between supplies onhand at thebegining and the end of the accounting period has been used during the period.

In other words your account balance says you have $310 of supplies onhand, when in fact you have a physical count of $190 onhand, so you need to bring your account balance into agreement with your actual account balance.

Your journal entry will be:
Debit Supplies Expense for the amount used (hint: difference between your two amounts)
Credit Supplies for the amount