You copy the url of the youtube (and only youtube) video. Not the embed link, the actual url. Paste it and put these codes around it:
[ yt ] URL OF YOUTUBE [ / yt ]
I have put spaces in the yt codes, erase those so there are none. The spaces are there so it doesn't disappear when I post it.
You cannot edit your post after posting it with a video in it. If you try and edit it it will make the video disappear. You also cannot click "go advanced" and click "preview" before posting, this has the same effect as editing. If you want to add something or correct something just double post after your youtube post, everyone will understand as we are all under the whims of youtube.