How to save a youtube video to your iphone library? No jailbreak?

I wanna post things on instagram of the videos from youtube please help thank you.

You can't. Simple. If you have a computer you can download the video using a youtube downloader then placing the file on your iPhone and accessing it under your "videos" but thats it.

People were recommending this program called ZillaTube. It has the feature to convert for your iphone device.

You can find it by googling for "zillatube.com", and download it from the first result top link.