The rooting itself isn't too difficult, you need to download the file, save it to an SD card, put the SD card into the phone then reboot the phone into boot loader mode and choose to load from the SD card.
Pretty much the phone does the rest.
Then you need to download the app from Google store and allow it to unlock the phone.
As above, warranty is out of the window if they find out.
If you use a shop to do it, and they do it properly, they will have a box that plugs into your phone to read part of a software file, this would then be sent to a server which calculates the code and returns it to the phone. It's not hugely difficult to do but sometimes the phone needs to be reflashed (new software loaded, so make sure you have a copy of all of your pictures/contacts etc) and it should take a couple of hours.