I only know the Mac/iPhone route...so if you have a Mac, here's my way:
Have the video file (I use .mp4) handy and download "Video to camera roll" on your iDevice. After that, connect your iDevice to iTunes and go to your device then click "apps" - scroll all the way down (the far right scroll bar) and then you'll see "Video to camera roll" on your computer. You can then open drag the file into the specified area or click where it says "add files" or something like that. After adding your video, it should automatically save to your camera roll. It should take a couple seconds when saving. Sometimes it'll be a split second and it'll say it saved, but it probably didn't. If that happens, you can just try to drag the file in again. From your camera roll, you can access your video! I use the "squareready for video" app to crop the video on Instagram.
Hopefully this made sense..let me know if you need more help.