To Edit your own post, click the little brown box at the bottom right that says "Email."
(I know, it makes no sense. What I don't know is how to change it. Deal with it...)
So, you've re-opened your post. Now you change whatever you like and when you're done, hit the little "Save" button.

To Delete your own post -- This feature hasn't been working lately.
So, we have three options. In all of them, you first edit and gut it, and leave a quick message.

1) Say "Please delete completely." After I've done it, no one will ever know it was there.

2) To get something like the deleted message above from "Seiyo," say "please delete content of this post." It will lose it's post number, which will change the numbers of all the posts below it.

3) If changing the numbers will create a problem, just gut it and leave some simple, inoffensive one-liner. This will leave the post number and the full signature.

to Quote someone else's message, click the little brown "quote" button.
To Multi-Quote a post (a fancy word for putting more than one quote in a single post)
click the squished up button in the middle.
If you're quoting someone like Gingobibble, a code in brackets will show:
Bracket - equal - Gingobibble - numbers - end bracket.
Then comes the text (Now is the time for all good men to come to the aid of their country.)
and the close quote: bracket - slash - Quote - end bracket.

If you don't mess with the codes at all, it'll look like this:
If you cut the name and numbers out of the brackets, it'll look like this:
If you want to make it easier for the rest of us to read, you can cut out
the parts of the quote that are not relevent to your response:
By the way, while reading a post as a Muggle, the Quote button and the
MultiQuote button are there. But that other little "quick reply" button is gone. ~ Psyduck