Transferring MP4's to iPad Air?

I have some MP4's of various films, and wanted to try and understand the best way to transfer one or more of them onto an iPad Air?
I've tried putting the vids onto iTunes, but can't seem to get that to work - but if I did how would I then get an MP4 that didn't ORIGINATE in iTunes (but is now stored there) transferred to watch on my iPad Air?
Thanks!
I've tried putting the vids onto iTunes, but can't seem to get that to work - but if I did how would I then get an MP4 that didn't ORIGINATE in iTunes (but is now stored there) transferred to watch on my iPad Air?
Thanks!