Log into your Store tab, then click on 'payment methods'. Under 'Custom Payment Type', type in Direct Deposit, or Bank Transfer or whatever you would like the option to be called. Then Apply Changes. This step will offer your payment type as an option during checkout to your customers.
If you would like to include your bank account details during the checkout process, click on the Store tab and then 'Checkout Settings'. Scroll to the bottom of the page, then in the box under 'Checkout Complete Message' you can add your bank details here ie 'If you are paying by Direct Deposit, please use the following bank details...' or similar. You can also add this info further up the page under 'payment message' if you like.