Tools needed:
18mm socket
ratchet
Flat head screw driver (of good size)
Drill
21/64 drill bit
Hammer
 

First remove your belt then unbolt the tensioner.  The tensioner is held on by just one bolt.  Now that you have it off.   Looking at the front of it as if it were on the car take the screw driver and stick it under the stretch indicator tab and start prying it up, you are wanting to remove the whole plate.  Try not to distort it to bad but it will need to be distorted some to get it off.  Once you get it off then pull out the bushing.  With the cover plate off and the bushing out you can pull the tensioner in half.  You may have to jiggle it a little bit to pull the spring out of it's holes.  Now you should have 6 pieces in front of you the cover plate, top bushing, top 1/2 housing, spring, bottom bushing, and bottom 1/2 of housing.  The next step is the most hit and miss part of the whole process. (Luckily I got it right the first time for my car) I have taken a picture with references on where I drilled my hole that worked for my car.  You might want to drill a few holes a just little bit a apart so you can have a fully adjustable tensioner.  The holes are going to be varies the location of the tensioner pulley and it's tension.  Now, that the whole is drilled reassemble the tensioner.  Bottom 1/2 of housing, lower bushing, spring, upper 1/2 of housing, upper bushing, then the cover.  The cover ease of installation is going to depend on how bad you distorted it removing.  Before I installed my cover plate I laid it on the some cement and pounded it flat with the hammer.  You may have to twist the tensioner some to get the tab of the cover plate to line up with it's detent (mine only needed a 1/4 inch twist for it to line up a little)  Then just take your hammer and tap the cover plate back into place.  Once it is held in place a little take the screw driver and hammer and go around the square hole that the cover plate holds on to and firmly hitting it to set the plate on good.  And there you go all you have to do is reinstall the tensioner and install the belt. You might half to remove and readjust the tensioner depending on you application.

   Belt tensioner completely disassembled.
 
 
 

This shows the hole I drilled in reference to the stock hole.  The upper whole is the hole I drilled.  The 21/64 beside the line is the drill bit size I used to drill the whole.
 

The picture below is what it will look like to run a shortbelt after doing the above modifications.  "Sorry for the poor quality, I'll take a pic of my own car next."

 
 
 

Another way I have seen the tensioner done to run only the Alt., waterpump, and crank pulley.  Is to use a carriage bolt and a nut to fasten the tensioner to it's mount.  Then you just move the tensioner to where it pulls the belt tight then you tighten the nut on the back of the tensioner mount.  Which will hold the tensioner in place.

I'm not sure how well that works.  I assume pretty well.  But I have mine done the way described above and it works like a champ.  Absolutely, NO, problems!