I was wondering if I could play half-life 2 mods with only portal 2 installed. If so how do I do that?

I don't know, but probably not. Mods are modifications of a source game, which means you need the source game to build upon with said mod. There's more mod-savvy people than me out there, but I'm sure you need HL2 to play the mods. In that case, just get it; it's like 10 bucks on Steam. If that's too steep, wait for Xmas.

No.
 
I think you can play some HL2 MULTIPLAYER mods, since that requires only the Source SDK and every Source game comes with it.
 
Do you seriously don't own Half-Life 2?

Funny enough I do have a boxed copy of HL2 on my shelf, but I attached it to the wrong steam account (as in a steam account I have lost all login details for and last used about 5/6 years ago) and I played eps 1 & 2 via orange box on 360 when I was between computers as it were. But it means, short of digging up my old account from my dead email account, I can't use it. 
 
I was just hoping that having the source sdk via portal 2 would be enough to play them. I had this in mind specifically. http://www.moddb.com/mods/the-stanley-parable/downloads/the-stanley-parable-v12

Shouldn't TF2 also provide the SDK? Now that it's free everyone should have it.