Okay, well, to answer your first question: This isn't entirely legal. Obviously, you're bypassing restrictions put on the phone to use cellular towers without a paid subscription. That's theft of service, and each carrier devotes a different amount of effort into pursuing cases like this. However, the cell tower may register this as a "roaming" signal, so it would be the same as someone with a T-Mobile plan calling in an area and getting service of the Verizon tower. I'm not exactly sure about that.
As far as the process goes, you basically just need to open up the inside of your phone and line up the transceiver chip. This should fit pretty snugly against your circuit board. All you need to do is make sure the metal on the chip is touching the metal on the transceiver in enough places - it's kind of like shorting electronic devices out - this will bypass the restrictions on the circuit board and allow the transceiver to run. Phish says you don't need to solder, but I always found that taking any wires running off the circuit board and solder them so they're touching the appropriate contacts on the transceiver helps too, that way the contacts on both chips are active. You don't need to do this (as the contacts should be touching anyway, powering the transceiver), but if you drop your phone or something, this will keep it from losing contact. This should still be thin enough that you can close your case back up, but sometimes you may need to glue the case together, as it may be 2-3 mm wider (that really depends on your phone).
Getting the phone apart is going to differ by model. what I've always done is take the phone apart from the headset jack - if you tear the little rubber cover off, you can usually just brute force the two halves of the cell phone apart.
For the transceiver, I recommend a USB-powered wifi card. They're usually the smallest, and that way you know you won't cut any unessential pieces out of the transceiver's board trying to fit it into the phone.
One thing to keep in mind, though: If you have your phone under contract (or recently, where the company will still have your serials), cellphone companies can sometimes monitor for this. To make sure that the serials can't be traced to you, either buy an unlocked phone, get a phone to unlock from a pawn shop, or just steal a cell phone (claiming it at a lost and found usually works).
Good luck!
