On your roku player go to the youtube channel and then open click until you bring up the red menu.
Then go to settings, and then scroll through and select pair device. Then make you're signed into the same youtube account on your tablet or phone.
If your tablet or phone is connected to the same wifi connection you should see a little tv like icon with a wifi logo on it.
Tap that and then tap connect to your roku.
You should then be able to browse videos, and then click play and they should play on your tv.