With the video pulled up on YouTube, click the share button, click options, click the "Long Link" button. Copy that.
Go to the post here that you want to make and paste it where you want it to appear in the post. For example, posting this:
http://www.youtube.com/watch?v=C-oDw2iVcTk
Will place it like this:
One shortcut is to click the address in the address bar of the browser for the YouTube video and copy that. But review and edit it as needed to begin with http not https and remove other data in the address such as "&feature=related". For example:
https://www.youtube.com/watch?v=C-oDw2iVcTk&feature=related
would become
http://www.youtube.com/watch?v=C-oDw2iVcTk
To display the video you see above.