Ok the first one I tried - took 3 weeks waiting for all the replacement bits I managed to break, the one I have just done took just over an hour.
When you order the case + Tri wing screwdriver, order a new touchscreen (if you are changing the colour of the case the suround changes colour and it is virtually impossible to remove this from the existing touchscreen without damaging it).
Have a tray to catch all the bits that come out (springs from the L & R buttons when taking the back off)
When putting back together use the original tri wing screws NOT the cheap ones that are with the case, make sure you use the right size screws for each part or you will have the screws poking through the plastic.