Step-2 : You will have to alter the existing ledger accounts to permit interest calculation on them. The same operation will apply when creating a new ledger account.
Interest calculation on outstanding balance is allowed for any ledger account. You simply specify the interest rate and style of calculation. Nothing is required to be done for interest during voucher entry.
For New Ledgers
Gateway of Tally.ERP Ø Accounts Info Ø Ledger Ø Create
On the Ledger creation mode activate the interest calculation option and specify the style of calculation.