Get a box like this
Make sure the cut out part is big enough for your head and neck
Lay down with your head and neck in the opening of the box. Look straight ahead and make some marks for where your phone should be then trace your phone, and cut a little smaller than you traced.
Now you have an opening for your phone
Poke a hole in both sides of the box for your earbuds or headphones to go through.
Ta da! Your private theater is ready for use!
How to use:
Find a video to watch
Lay your phone on top of the opening. (That's why you cut it smaller than you traced, so that it won't fall through!)
Thread the earbuds/headphones through the hole and plug into phone.
Then you put the earbuds in and lay down like so, press play, and simply look straight!