How do I install this theme???

I just signed up for wordpress and am curious how to install a theme that I’ve found from another site? I know basic HTML and am not quite understanding what I’m doing wrong?
Here’s the site that has the themes:
https://scribblescratch.com/themes2/

I downloaded the theme I want to use and paid for the CSS upgrade here, copied the style sheet that came in the download, replaced the CSS sheet in wordpress and clicked on the “start from scratch and just use this” tab and the theme still isn’t showing up.

Can someone please tell me what I’m doing wrong???

Thanks so much!