Watching downloaded films on iPad

Going on a flight soon and would like to know how to watch downloaded films, last time I could watch a bought film but not the rented one, when researched upon returning and learnt that you should start the film to play while connected to Wi-Fi then it would be ready to watch in your library, I did this but could still not not watch as they were not in my library with not being connected to Wi-F, I am now at a loss of what I need to do?