Well it depends if you want to post your own clips or link to someone else's. I've posted up 170 of my own clips and I think I've linked to 3!
Anyway it's very easy.
Find a clip you like - in this case I'll use a random one of mine, Eric Morris from This Is Ska.
On the right of the viewing screen you'll see the URL, it looks like this:
https://www.youtube.com/watch?v=OwCOou0SejI
so what you do to make the clip appear on your actual page is to take everything that appears after the equals sign in the URL, in this case
OwCOou0SejI
then all you do is type this: (actually not showing up but it's one of these [ followed by the word tube then closed by one of these ], add your piece of the URL then another of these [with a slash immediately following / and then the word tube again and then close it with another ]
and in between the two, you add the part of the URL that you save, like this