How do I make a video clip fit in this big green screen from a certain youtube video?

You need to motion track your screen content footage into the greenscreen footage.  Try this tutorial.  

https://www.youtube.com/watch?v=q85hui5DSqI

Hi JBJB1029.

Thanks for writing in.

Here's a YouTube video that should help: https://www.youtube.com/watch?v=nrwf0wHts7c&ab_channel=MotionArrayTutorials

Let us know how it goes.

Thanks,

Nishu

Start by actually educating yourself about proper corner pin tracking eitehr using AE's legacy 2D tracker or mochaAE. There's literally hundreds of tutorials on YouTube on that and even the worst ones will produce better results than what you currently have.

Mylenium

May I know what have you tried that is not working? It should be quite a simple process with tons of tutorials on youtube. You can try keying off the green and put it over a comp with the footage with corner pins applied.