Double-entry bookkeeping, with a couple of exceptions.
If you look at a bookkeeping textbook, you may get the impression that double-entry bookkeeping is the only system in use. Actually double entry is only suitable for professional accountants, and most small businesses would do best to look at a simpler system.
Of course, you may take the view that if your business venture falls through, then you would look at a career in accountancy. That is one exception. You may already be using a double-entry system, probably as computer software, you are happy with it, and would like to know more about it. That is the other exception.
If you want to be an accountant, then of course you will need to learn double entry. After that, you might take a look at debit-positive convention. This is where debits are treated as positive numbers, and credits are treated as negative numbers. This gives a compact method of dealing with accounting transactions which is useful if you have to deal with anything that has to be consolidated, or if you just want to present a schedule where you can sight-read all the numbers. It is also possible to work in credit-positive convention.
The method of entering numbers on the David Porthouse & Co spreadsheet owes something to debit-positive convention. If you type a negative number, it is transferred to the other column (this works with credit-positive convention as well). This allows numbers to be entered quickly from a handheld numeric keypad if things need to be done manually.