You put your comments directly after the text you comment upon, making the context of your text clear. You leave blank lines between the quoted and your own material, allowing readers to find your text quickly. You don't leave blank lines within quoted material. They make it harder for the reader to understand the context and understand what you are writing. Fine points When you quote a message that quotes another message, make sure to preserve the attributions for all the included quotes.
Some people complain that the attribution here is misleading.
You may want to move the attribution for Q to the first quote of Q's text. OTOH, readers usually look at the top of the article to find out who wrote something. One way to reduce the chance of misleading readers is to put an ellipsis after the attribution. You may be breaking this rule without even noticing, so read on. This automatic line breaking is particularly bad when applied to quoted material.