This tutorial should only require you to have owned and installed the game you want to play + have TE120 installed and ready to play.
You'll also need GCFScape[www.nemesis.thewavelength.net], a free tool used to open up the vpks inside various source engine games.

If you're file-savvy and know how to access the steamapps folder, you can skip these below steps.
Otherwise...
1. Go to your library and right-click on any game. Click Properties...


2. A window should pop up. Click on the LOCAL FILES tab on the new window, and select BROWSE LOCAL FILES.


3. Head back to the steamapps folder from your game folder.

1. Access Transmissions: Element 120's main game folder from
steamapps/common
OR
steamapps/sourcemods if the version of TE120 you have is not from steam.
Open up the gameinfo.txt inside.


2. Once inside the gameinfo.txt, add:
game+mod |gameinfo_path|custom/*
under the line:
SearchPaths
{
Do not mess with anything else in the text file.


3. Go back to the main game folder and open the te120 folder. Create a new folder inside there, and name it custom.


NOTICE: This is to ensure that we have a place to put files in/take files out easily, while making sure that TE120 will read those new files.

4. Go back to steamapps/common and open up the Half-Life 2 folder.

1. Select and open the:
hl2 folder for Half-Life 2;
episodic for Episode 1;
ep2 for, well, Episode 2;







2. Open either the hl2_pak_dir, ep1_pak_dir, or ep2_pak_dir vpk with GCFScape.
IGNORE any other vpks, including hl2_pak_000.


3. Drag the maps and scenes folders in GCFScape onto a new folder in File Explorer with an appropriate name. (EG. HL2 Maps)
Place the named new folder inside TE120's custom directory if it isn't located in there.


NOTICE: Why do you need the scenes folder, you ask? Well, it contains the needed scenes for NPCs to correctly play out their scripted sequences, otherwise you'll have to noclip your way through levels due to the AI not playing out their scripts.

4. We're not done yet!
That maps folder doesn't have the game of your choice's maps, but graphs used for the AI. The graphs are essential to get the AI to function properly, or else the AI will always be disabled in their maps.
Close GCFScape and copy the maps folder in hl2, episodic or ep2 to the folder you named in TE120's custom directory.


NOTICE: Don't worry about the unfamiliar maps and whether they appear or not; those are some custom maps I had made with the Hammer Editor.

This section requires that you have successfully done the steps listed in the How-to section, with the appropriate game of your choice for the appropriate step you'll take here.

Launch TE120, and open up the developer console using ~ or whatever button you bound the console to. Click here to find out how to enable it.

== Half-Life 2 ==
Type "map d1_trainstation" into the input box (without quotation marks) and you'll get a list of d1_trainstation maps.
You can choose either d1_trainstation_01 (the very start of HL2) or d1_trainstation_06 (just before you get the crowbar from Barney).

== Episode 1 ==
Type "map ep1_citadel" into the input box (without quotation marks) and you'll get a list of ep1_citadel maps.
You can choose either ep1_citadel_00 (the very start of EP1) or ep1_citadel_01 (after you get off the van and you and Alyx proceed).

== Episode 2 ==
Type "map ep2_outland" into the input box (without quotation marks) and you'll get a list of ep2_outland maps.
You can choose either ep2_outland_01 (the very start of EP2) or ep2_outland_01a (after you get the crowbar and fall into the mines).

Refer to the give physconcussion section to find out how to get the special gravity gun.

This requires the developer console. Once again, click here to find out how to enable it.

1. Type "give physconcussion" into the input box. That's it.

This method is as best as it can be without outright editing and compiling the source code of TE120, and I have, unfortunately, not tested this with either of the Episodes, though I suspect EP2 would have the least bugs as TE120 appears to be built on the branch of the Source Engine EP2 uses.

There are still problems you can encounter in the maps such as lack of tau cannon on jeep, so beware.

When you want to return to playing normal TE120 maps, you need to close the game if you haven't already, and move all folders containing HL2, EP1, or EP2 content from custom to somewhere else.

Scenes from the custom folder will override the vanilla TE120's scenes, although this seems to only mute G-Man's speech at the end, and does nothing else gameplay-wise.