You mean how do you make one, or post one.
You make one e.g. by pressing Alt+PrintScreen buttons when the window is active. This will put the image to the clipboard. You can then start your favorite drawing program, insert the picture from the clipboard (Ctrl+V usually) and save it (ideally as JPG or GIF or PNG).
To post it to the forum, just use the Attach field in the 'Post Reply' form on this forum.