1. Go to Steam\steamapps\common\left 4 dead\left4dead and open pak01_dir with gcfscape (https://valvedev.info/tools/gcfscape/)

2. Extract the files to the 2ND left 4 dead folder (not the 1st) 

3. go to the 1st left 4 dead folder and make a folder however you want to name it (i picked left4dead_Mods) 

4. go back to the 2nd left 4 dead folder and open gameinfo 

5. type "Game (press tab 4 times) and type in the folder you named 

6. download whatever mod you like (some l4d2 mods work)

7. open the mod VPK on GCFscape

8. extract the folders to your mods folder 

9. all done: 

Forgot to say, rename pak01_dir to something simple, like pak01_dirA