Please help with some basic coding confusion

I’m busy learning CSS and html as I go.

I want to know from some of you experienced and great coders, how would I for example edit the CSS of a single page, without using the wp page specific css plugin?

I would like to have all of my edits in the stylessheet in my childtheme, so that when I make any theme or plugin updates, they remain the same.

For example I want to edit *this page:
http://peopleofthesouth.co.za/bypass (bypass coming soon mode)

*http://peopleofthesouth.co.za/valleyfamilyforum/

But instead of doing it in that pages css (which im not even sure where it’s stored) I want to make it in the child themes css file.

Please help this aspiring young coder 🙂