You can watch the manual installation process in this video for chrome (windows): https://www.youtube.com/watch?v=SffjcBh520w You need to download the files here https://github.com/ducksper/lichess_enhancement/archive/master.zip . Then you dezip the folder. On chrome you go in the extension section, and you activate developer mode. Then you import the folder by clicking on "install unpacked extension chrome".
PS:The script may have performance issues at the end of games so it may be better to avoid bullet games (especially on Firefox).