Here is a diagram showing where the orifice tube is located.
The orifice tube is in approximately the center of the image. It is located inside the lower pipe of the a/c evaporator. Basically just remove all r134a from the system, pull the connection apart at that joint with the special a/c hose removal tool and use thin needle nose pliers to pull the old orifice tube out. Make sure you don't break it apart as they can be extremely brittle after years of sitting in there. You may need to lubricate it to get it out. I don't know which lubricants are compatible with r134a so I would just squirt a little PAG 150 oil in there. PAG 150 is the standard lubricant for the a/c system anyway, and you can get a 3oz bottle of it at autozone for $6.99.
Once you get the old orifice tube out, lubricate the new one with the PAG 150 oil, and push it back into place approximately in the same position the old one was in. Using the new o-rings that the orifice tube should come with, re-install the a/c hose and then go through the steps of vaccuuming the air out, testing for leaks, and refilling with new r134a.
Autozone has kits for special order to help you remove the old orifice tube but I found that a little lubricant and some needle nose pliers worked just fine. I was too rough when i was pushing the new one in and I broke it (it can be a really tight fit), so I just went to the store and bought another one since they are only two bucks.
I take no responsibility if you follow my advice, and I don't guarantee that it is the proper way of doing it, but this is how I did it and it worked fine for me. This does not cover flushing the system to clean out the rest of the crap in the lines (which I highly recommend) since I replaced everything except for the condenser and evaporator.