Trading by cash is one the most used method in shops and discount centers. to keep a cash book updated and write a cash book in true format, you need details of cash flow.
When Cash receives from customer, it will be added to cash account with amount of it in Debit column
When Cash be paid to Vendors, it will be added to cash account with amount of it in Credit column
Under view of accounting concepts, when you receive cash, cash will be debit and capital will be credit and vise versa.
if you want to have automated cash book from your invoices, so you need to use an accounting software but if you are using paper system, so that you should be careful to do not make a mistake in writing voucher in right place in ledger book.