Just a tiny addon that lets you turn the maps into a playable sourcemod. You will still need to download the map file, but follow these installation instructions instead.
INSTALLING THE ADDON
Step 0 (optional): If you previously installed the maps in your common/Portal/portal/maps folder, you can delete them if you wish (they're the ones with "_d" suffix). I recommend verifying your local Portal game files either way, just in case.
Step 1: Download the maps file and this addon file. DO NOT extract the maps yet.
Step 2: Extract this addon file to your SteamApps/sourcemods folder. (Should result in a folder called "Portal Reversed" and a readme file that mostly repeats this description.)
Step 3: Extract the maps file to the Portal Reversed folder. Drag the graphs folder into the maps folder.
Step 4: Restart Steam. Portal Reversed should now appear in your games list.
Step 5 (optional): A custom background file ("custombackground.png") is included in the Portal Reversed folder for usage in the new Steam library if desired.
FEATURES
-crosshair is (re)enabled whenever you start a new game
-playable as a mod
-chapters, chapter titles, chapter images, game icon
-honestly it's just a basic mod addon, I'm no amazing dev, I just like playing mods without messing with the main Portal folder
KNOWN ISSUES
-The loading window uses #Incorrect_Placeholder_Names instead of the map name. I don't know why. I tried to fix it, but nothing worked. (Downside of this being my first basic modding attempt, probably.) Sorry! If someone knows how to fix it, let me know and I'll update the addon.
-escape_01_d appears to have shipped with a missing cubemap. This is not something I can pre-fix, but if the pink-and-black-shaders happens to you, you can easily fix it by 1) making sure your game dimensions are larger than 512x512 and 2) typing "buildcubemaps" in the console.
-Chapter 12 is intermittently unselectable, at least for me. I don't know why. When it happens to me, it also happens in other mods with more than 11 chapters, which leads me to think it's either an issue with Valve or an issue with my Portal files. If this happens to you and you need to access it directly, use "map testchmb_a_01_d".
i can't start a new game. What i can do?
What specifically is wrong? Like, is the New Game option missing, or does it not load when you select a chapter, or what?
you have to download the maps rar (first link in files that says instructions) and extract the maps and graphs to their respective folders.