In the end, your installation should look something like this:
- Download Team Fortress 2 and Source SDK Base 2013 Multiplayer, they have to be on the same drive. Source SDK Base 2013 Multiplayer is located in Steam under Library > Tools (watch the gif for help)
- Download the main LF:E 4.1 files and the 4.1 hotfix
- Unpack the main 4.1 files first and place the tf_coop_extended folder into your sourcemods folder. Usually the sourcemods folder is located in C:\Program Files (x86)\Steam\steamapps\sourcemods. If the sourcemods folder doesn't exist, create it. After that, unpack the hotfix files and replace the existing main tf_coop_extended files in the sourcemods folder with the hotfix files
- Restart Steam and you should be able to see Lambda Fortress: Extended in your Steam library!
https://github.com/stoneman2/tf_coop_extended