I'm not sitting at the facilities at the moment, but, I do this frequently with Yotube. There is an icon, top right, which you click and the rest is automatic.
ASnother member might be able to detail that a little better than I.
I also cast from my tablet to my TV, with no problem at all but when I tried this from My5 on the tablet, the casting icon did not show on the tablet. Could this be to do with updating the My5 or something on the tablet?