How to put a fully horizontal video on instagram without being cut?(iphone)?

I know you can screenshot a photo sideways but i see some people having horizontal videos that fit in instagram, how do they do that?

I think you need an app to do that… An app for video making

NO

You will have to compose your shots so that you can later crop them to a square image aspect ratio.

If you do not have a computer with a photo program on it, you may want to look for the app, Photoshop Express.