No one really had a guide on this site that I could find so I figured I'd post one.
It is really not that difficult. Trust me, if I can do it, so can you. I am no expert at circuitry.
Anyway here we go.

I recommend that you wire your fans in parallel. Basically all that means is if one fan goes out, the others will continue to run.
It's going to look more messy than if they were in series, but it's better than an overheated grow space.

Firstly, you need to find a 12V adapter laying around that you don't use. I found one that used to charge an old remote control car.
See Pic 1.

Next you can either buy some computer fans, or strip them from old computers.
If you have access to a power supply that you dont use, those fans are actually very powerful for their size and move a good amount of air.
Pic 2 are the 3 fans I've chosen.
The big one is just a computer intake fan.
The little one is the cpu fan.
And the middle one is the powersupply fan.

Next, we need to strip the wires so that the core of the wire is exposed.
See Pic 3

Now the easiest thing to do from here is to connect all three of the blue fan wires together by twisting their cores together and making a point.
Now connect that with the wire from the adapter that is plain black by the same method.

Now do the same thing with the red wires from the fans and now the wire from the adapter that has white stripes going down it.
*If it only has 2 wires, make "blue" = "black" in the guide.*
See Pic 4

From here it will work just fine, but if you have any electrical caps laying around (sorry for my lack of electrical terminology) cap the ends of our points.
See Pic 5.

Plug and play.
You can always extend your wires with other wires, but that is pretty self-explainitory.
Anyway, I hope this guide was informative, please post any responses you might have, questions, etc.
And of course, keep on growing.

Duece.

PS, Pic 6 is my baby mallard Doobius Maximus, or as I like to call him, Doobie.

so ive got a 12v 120mm fan i just bought from radio shack, i found an old AC adapter, its also 12v output. seems like a good pairing, but ive heard of cooking the fans from sending it too much power? if i use your set up and just run the one fan, will it be safe?

Oh yeah man you will be fine for sure.
As long as you don't go over 12V output.
You could run quite a few fans on that adapter if you wanted to btw.
Good luck mate

I have a few spare adapters kicking around and was thinking about putting them to use if i can.
the one is 12v @1000 mA so im sure i can use this with any standard 12v comp fan but my question is that 1000 mA seems like a lot for a little fan. Can i run multiple fans off this adapter without reduced power and if so, about how many can i run off it without losing power?
i have four other adapters that are from old cordless phones. they are all 9V output except one with different amperage. one is @ 200 mA, two are @ 210mA and the last one is 6V @ 100mA.
Is there any use for these adapters? and if so what would be the best way to use them?

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Hey shout, summertime blues, Jump up and down in my blue suede shoes

Use the 12V. Don't even mess around with the 9V or 6V
You should be able to run quite a few fans on it without losing any power.
As for your other adapters... nah, but don't throw them away. You never know.

Thanks for posting.

Later.