Turn it off, take the sim out, avoid turning it back on by mistake.
Unscrew the two screws at the bottom. use a suction pad (back of a sat nav or something similar) to gently pull the screen away from the phone. You can use a stanley knife or a thin screwdriver to do this too, but you might damage the rubber seal. I did it that way and it was fine, just a tiny bit of damage.
Anyway so SLOWLY pull the screen away and use a screwdriver to pop up the 3 connectors at the top of the phone. 1 you need to flip up a little lever to slide it out, it's easy though.
once thats out, put the phone part to one side. Take the screen part and unscrew all of the screws. They are all the same size (apart from the ones that hold the screen to the phone. So don't worry too much. For some you will need to peel back some tape.
The next step you have to be careful with. This is where you lift out the LCD screen. There is an aluminium frame holding the screen in place, gently pull these with a stanley knife to the side so the screen can pop out. Careful with it, don't let dust get on it.
Then you are left with just the cracked glass on the plastic frame. Use the stanley knife to remove all of the glass, except the home button. Stick the sticky pads down on the plastic and put your new glass on top press it firmly and reverse the process I just told you.
It took me about 30 minutes.