This is the first comment.

This is the second comment, a response to the first comment. It is achieved by adding a colon (:) to the beginning of the comment. Each new line you insert needs a colon at the beginning to indent this way.
This is the third comment, a response to the second comment. It is achieved by adding two colons (::) to the beginning of the comment.
This is the fourth comment, a response to the first comment, again. Note how this returns to one level of indentation.
This is the fifth comment, a response to the fourth comment. By indenting one extra level, you indicate that you're replying to the person above you.
This is the sixth comment, a response to the fifth comment. This one has three colons (:::) at the beginning.
This is a new response to the first comment.
This is a response to the above.
This is a also response to the above.
This is a counter-response to the above.
This is a counter-counter-response.
And so on and so forth.
This way, threads can be easily identifiable.
You can hit edit and play around here until you get it if you like. Once it clicks, it will be easy and more-or-less intuitive.

This is an outdent, to bring the text blocks back. The code for this outdent is {{Outdent|::::::}}. Notice how the number of colons inserted after the pipe (the '|' character, achieved by hitting Shift+\ on your standard keyboard) is equal to the number of colons of the line it replies to, instead of being one more. In this case, it is telling the template how far indented the comment you are replying to is, while your own comment gets indented back to the beginning. This can make things much easier to read in long discussions.

Each comment should be indented one level past the comment it is replying to. So when there are two replies to the same comment, they will have the same indenting. Always place your own comments at the end of a thread, to help others get the order in which the comments were made.