A Letter of Instruction is a letter that gives another party order or authority to carry out specific instructions. A Letter of Instruction to Bank is a document that sets out the specific instructions and expectations for how the bank should operate with respect.(This form must be sent fully completed – typed or in capital letters - and signed to. Obtain a letter of credit application from the bank and fill it out with the relevant information. Direct Debit Instruction to your Bank or Building Society. How to close a credit card account. What is your routing number? Do you know how to find out if you are a beneficiary on a bank account?