Well, if you're using Windows, there's not much to it. You can't attach a file, you have to copy & paste.
Open your document.
From last word of the document, hold down your left mouse button while you drag the mouse back up over the document (to select the portion you want to copy).
Release the mouse button (text should remain highlighted).
Press the right mouse button and select "copy."
In the thread you wish to reply, click the button "reply"
Click inside the big white block to the right of "Your Reply."
Press the right mouse button, while keeping your cursor inside the box.
Select "paste."
The message board limits the size of file you can post. Best to avoid graphics - just post a web address instead.