Tools required: iPhone opening tool, Phillips size 0 screwdriver, needle/drawing pin/paperclip or something similar
First remove the sim holder. To do this, push something like a needle into the small hole at the top of the phone. Push down and the sim holder will pop up.
The sim holder should now be removed.
Use the opening tool, or something strong with a thin edge (something plastic is best because a metal tool will damage the case), to pull the antenna cover off in the direction of the arrows. Some force is required.
You should now have the antenna cover removed.
Remove the three screws, circled below.
Run your opening tool around the join of the back cover. Some force is required.
The back cover should now be loose. Carefully lift it away from the phone. It will still be connected by a ribbon cable. The connector (circled) attaches like a plug and socket. Lift it up and it will unplug.
You should now have the back cover removed. Now remove ten screws around the outside edge of the phone.
With the ten screws removed you can remove the silver bezel, as shown below.
Your phone should now look like below. Three more connectors need to be disconnected (circled). Do so by using pushing a small screwdriver underneath then levering them off. Remove the three screws, indicated by the arrows.
Now pull the circuit board away, like below. It'll still be connected by some more wires so don't try and remove it completely. The battery now needs removing. It is held in place by some very tacky glue. You need to try and slide something underneath the battery to lever is out. An old credit card cut in half should do the job
The LCD screen and digitiser are still held in place with tacky glue at the top and bottom. If possible, use a hair dryer to heat the glue up to make it easier the remove the screen. Then push something like a credit card in between the screen and the rest of the phone to help release the screen. Once the screen is loose, turn the phone over and you'll notice a few ribbon coming away with it. Ease these through the back of the phone so that they don't get damaged.
You should now have the LCD screen and digitiser removed. Reverse the whole procedure for reassembly.