I thought the general rule of thumb was, when you read it back to yourself and you take a breath, you either put in a full stop, or a comma. No doubt you will be pouring over this message of mine, for punctuation and grammatical errors
The pause rule, or breath rule, for commas works about half of the time. Commas, essentially, separate main parts of a sentence from lesser parts, so to use commas properly, you need to be able to identify the parts of a sentence and understand how those parts are working together.