you go to youtube go to a vid u like and look where it says ADRESS or WWW.YOUTUBE.COM.and click your left button.if you did it right it will say cut copy paste.if it doesnt click the other button.always copy.then paste.
well, the way i do it is first go to youtube (it doesn't seem that you can do this with other websites) find a video you like there. then under the vid it should have a button that says share click it then it should have a highlighted bunch of letters right click it then copy then click 'embed a video' back here and paste. simple.
here's a aleatório vid that i embed just to show it works.