Function to show a custom field review score over thumbnails

Hi,
I’d like to add a function that adds a review score over thumbnails on homepage and archive pages.

I want to:
1. create a simple custom field with the score
2. output its value (for example “7/10”) via a function
3. style it with css

The problem is I have no idea how to do the 2nd point. The function would need to get the value of a costum field and output it on top of the thumbnail. I don’t have any knowledge about writing functions. I usually just “copy/paste” that kind of codes.