Comments - easton9

Thanks malorie for the solution! It was fun playing this mod :)

To sum up what worked for me:
1. install Half-Life 2: Episode One
2. install Source SDK Base 2013 Singleplayer (Library --> Tools)
3. right-click on Source SDK Base 2013 Singleplayer, in Properties go to the "Betas" tab and change "Select the beta you would opt into" option to "incoming" (Steam then downloads aditional files)
4. put the "sebastian" folder in Steam\steamapps\common\Source SDK Base 2013 Singleplayer
5. apply this fix (Sebastian Patch - Source SDK 2013) to mod folder
6. go to Steam, right-click on Source SDK Base 2013 Singleplayer, in Properties go to "General" tab, click on "Set launch options..." and enter the following: -game "sebastian"
7. restart Steam
8. launch Source SDK Base 2013 Singleplayer, the game should be running now!

If there is any error, verify the Source SDK Base 2013 Singleplayer cache in Properties, "Local files" tab.

Hopefully this resolves problems for everybody else, too ;)