Create a portal 2 mod
Although the "Create a Mod" option of the Source SDK is not included in the Portal 2 Authoring Tools beta, you can still create mods.
The Method is a simple method, but takes a bit of pateince, due to 9.45GB being copied.
How it is done
First, go into "C:\Program Files\Steam\steamapps\common\portal 2\" or whatever your Portal 2 game files folder is
(the "Program Files" may be "Program Files (x86)" or "Program Files (x64)").
Highlight the "portal2" folder, and copy. (Ctrl+C or Right click and 'copy')
Now go into "C:\Program Files\Steam\steamapps\sourcemods" or whatever your SourceMods folder is.
Paste the files and wait for them to finish copying.
Now, you are free to edit any of the Portal 2 files (in the sourcemods folder) and create your mod.
To install the mod on other computers it is as simple as downloading the mod folder and extracting it into the sourcemods folder!
The Portal 2 Steam appid is 620. On the gameinfo file, change 'title' and 'game' from "PORTAL 2" to whatever your mod is called.