Mods normally work better (and safer) inside the Custom folder; you should be able to find it in Steam/SteamApps/Common/Half-Life 2/HL2/CUSTOM. If such folder does not exist, then create it.
1) Inside the Custom folder, create another one named HL2RDM.
2) Inside HL2RDM, create another folder called CFG.
3) Inside CFG, create two text files; one named skill.cfg, and the other skill_manifest.cfg (remember to erase the .txt part at the end, since Half-Life 2 works with .cfg files instead)
4) Copy this code, open skill.cfg, paste the mentioned code there, and save the file.
5) Open skill_manifest.cfg and copy the text below:
exec skill.cfg6) Save the file.
7) Run Half-Life 2, Half-Life 2: Episode One, or Episode Two, and make sure "normal" difficulty is selected under OPTIONS menu.
Delete the HL2RDM folder from Steam/SteamApps/Common/Half-Life 2/HL2/CUSTOM.