how download youtube video into iMovie to edit

Yeah, so basically, I want to edit and comment over animal videos, or really, just funny or neat videos for my 4 year old niece who I don't see as much as I'd like to. she watches YouTube now as a treat and the videos are weird. Yes, I could and will make my own videos but I'd like to start with this. And nature videos with animals doing funny or cute or interesting things is where i'd like start but it's the "how" that I'm interested in. Literally how do I do it. Yes, I have quicktime.