How to mix original instrumentals?

Does anybody know how to do it?

You know what I mean!? Like Shane Dawson does it in his spoof
songs*...because YouTube doesn't let you use original music, you have
to make your own, so how can I do it...how can I change the original
instrumentals of famous songs...
I can download the instrumental online, and then what can I do with
it??

I'm not talented enough to make my own music, and because I want to
make music spoofs like Shane does, that music has to be as similar as
possible to the original songs.

* http://www.youtube.com/watch?v=PaVQIu7i1Lc

http://www.youtube.com/watch?v=XrAuEZwqR0o

http://www.youtube.com/watch?v=9C3kHl-V4dY

see how his music is almost the same as originals....
now, ok, I know he has people who do the music for him....he doesn't
do that on his own, but still...I doubt that those people start from
scratch and end up with such good instrumentals, almost as original