2021 Edit: Much shorter/concise install guide by Jaio

Original stuff I wrote below:

Video guide:

Text guide (from video description):

If any of the links get deleted for some reason, please DM me on Discord: ImAciidz#4686

So, if you've tried this mod in the last ~3 years or so, you'd know that the elevators don't work after chamber 04 or so (don't remember, and it doesn't matter specifically). This is caused by an update done to Portal 2 at some point since the last runs were submitted/verified in December 2016. As a result, the only way to play the mod is to download an older version of Portal 2 (legitimately) which I will show how to do in this guide. I will also show how to mount the P1 Done P2 content onto this older version of the game so that you can add the older version as a steam shortcut and play it that way.

Prerequisites:

Windows, as I do not know how to run DepotDownloader on Linux/MacOS. (MacOS doesn't even have 32-bit support anymore, so you won't be able to play the older version if you're on it anyway)

.NET 2.x (2.1.22 at the time of writing, or newer): https://dotnet.microsoft.com/download/dotnet-core - get the Windows x64 installer in this section: https://imgur.com/a/jrC7JAq

DepotDownloader (2.3.6 at the time of writing, or newer): https://github.com/SteamRE/DepotDownloader

Optional depending on later choice: Python 2.7 (2.7.18 at the time of writing): https://www.python.org/downloads/release/python-2718/

MEGA Link (read below before downloading): https://mega.nz/file/IFVCVRLa#ezrgnXD8Q1Z871PpWo5z1oEUar176uq-x6a_BO7TDS8

Original download (read below before downloading!!): https://steamcommunity.com/groups/p1donep2official

So, first of all, get DepotDownloader, and extract it to a folder. Click in the file explorer's address bar and type cmd, then hit enter. After this a command prompt window should be opened to the folder you extracted DepotDLer to. Type dotnet DepotDownloader.dll in the cmd prompt and hit enter. If it errors, you need to install .NET 2.x. Once you’ve got it working, make a folder where you want to download the old version of Portal 2. Copy and paste the commands below, replacing -dir xxxx with -dir “C:\exampleDirectoryHere”. If you don’t do this and end up having issues I will not help you.

Once the game is downloaded you can delete all of the alternative language folders unless you specified depotdownloader to download another language because...well..you did. You can also delete the maps folder and save ~2.5GB of space.

You now have two options: download P1DoneP2 from the official source and run the python script (this requires that you have Portal 1 installed on your computer) or use my upload on MEGA which has the proper textures, and doesn’t require you to have python or Portal 1 installed on your PC. I get the point of the python script is to negate piracy or anything like that since it requires you to actually own Portal 1 to play this mod, but the thing is, Source Unpack has been publicly hosted for anyone to download for years regardless of whether they own the games included or not, so I don’t see how this is any different. At the end of the day, you still need to own Portal 2 to play the game and see these textures, so I don’t see an issue.

Either way, once you have the resulting p1donep2 folder from either my download or the Python script, just cut and paste the folder into the same folder you named in depotdownloader to install the game. Rename the p1donep2 folder to portal2_dlc3. At this point you should be able to launch the game and play.

dotnet DepotDownloader.dll -app 620 -depot 621 -manifest 4599777261270266522 -dir xxxx -username xxxx

dotnet DepotDownloader.dll -app 620 -depot 622 -manifest 1640119522092278551 -dir xxxx -username xxxx

dotnet DepotDownloader.dll -app 620 -depot 624 -manifest 9067046041888467657 -dir xxxx -username xxxx