Matting/Mounting a Portfolio

Im auditioning for an art school in the city, and they asked for a portfolio and stated that:
Portfolio should include between 10 – 20 pieces of work.
All art pieces should be either matted or mounted.

As far as i know, matting/mounting a piece of artwork is to secure it in cardboard/foam, but im afraid it would be too thick.
Any tips on matting or mounting or the process?
I'd like my works to still be thin enough to fit it in a folder, is there a way to do that?

I don't know if it is too late, but if you haven't done the portfolio yet, I think these videos could help : )

how to mount:
Help with layout, fonts and more:
some tips on what to put in there:

just make something professional looking in a folder, they will probably look at your stuff, even if it is not mount exactly like they would have wanted it: )

Thanks for your splendid advice lokken! Those are awesome videos, super helpful and thorough!

Thank you alot! This is very useful!