I spent ages trying to figure out how to make trailers from my IPad, and it took me forever, but I found a way.

Go to the App Store and download an app called Video Downloader Super Lite. There is a paid version you can get if you want, but I wouldn't recommend it.

Open up the app (YouTube should open automatically), and find the clip you want to download.

Once you have found it, there is a button at the bottom of the page that says 'Save'. Click on that, and the video should be moved to the tab at the bottom that says 'Downloads'. Click on that, see if you have enough file space (if you don't it will say and won't download), and it will start to load. This may take a while, so bare with, but once it has loaded, click on the blue arrow next to it.

It will say 'save to memory' and click the button underneath that, it looks like a camera. It should then be moved to your camera roll.

NOTE: if you don't have enough space on your device, the file will not be exported.