1. Highlight the url in the address bar on the page of the video you want to embed.
2. In the line of icons just above the white field where you type your post, look for the icon that looks like a filmstrip. You can hover your cursor over it and a little pop up will say "Insert Video". Click on that and paste the URL into the box and click OK.
3. Click "Post Quick Reply".
4. ??
5. Profit.