The most convenient userscript for me is this one that automatically likes YouTube videos. It's configurable to be able to: like the video after a specified watch percentage, ignore already disliked videos, only like videos from subscribed channels, and ignore livestreams. I like it enough that I've made a few pull requests to fix it when YouTube changes their UI.
When I have the time, I work on an in-progress local version to implement a few new features including: (1) Support for the YouTube shorts UI. (2) An option for a notification/toast to appear when the video has been liked. (3) An option to check the watch percentage continuously (mutation observer) instead of a user-defined poll rate which sometimes misses liking very short videos in playlists. Eventually I'd like to port something like this as a YouTube ReVanced patch.
I always remember the difference by imagining them as lines on a graph. Forward slash is the line with positive slope and backslash is the one with negative slope.