Answer
Total both sides of the account. Insert the balance c/d on the smaller side to make the totals equal, rule off with equal totals, then bring the balance down (b/d) on the opposite side below the totals as the opening balance. The b/d side indicates whether it's a debit or credit balance.
Dr 1,200 vs Cr 900 -> c/d 300 on credit side; b/d 300 on debit side (a debit balance).Real-world example The bookkeeper balances each account this way before listing balances in the trial balance.