This is the bit that most people don't know what to do, so from this point forwards I'm going to split into two sections:

Section A) Computer/ Laptop

Section B) iPad/ Phone/ Tablet

As they are very different, and really need to be talked about separately.

I will start with Section A:

Open up your Internet browser, and type into the search bar:

snipmp3.com

When the website opens, it will have a small bar for you to insert a YouTube link.

Go onto YouTube, find your selected clip, and copy the URL (the bit in the bar at the top of the screen).

Go back onto snip mp3, and paste the URL into the bar on screen. Wait for the thumbnail to load, then click the download button (I think it says import on some), and your clip will start to download into your files. This may take a while, as this website runs on JavaScript, but be patient it won't take that long.

NOTICE: from my personal experience I have pocked up that for some reason, I don't know why, this website won't let you download VEVO videos. It may just be my computer, but it also did it to my library computer, and I have not yet found a way around this, so you have to find a copy of the video on a fan account or something like that.