The 1st thing you want to do is find out if it is an iPhone 3G or 3GS. The glass digitizers have different pin connections. Make sure to order the correct Digitizer.

Make sure to completely shut down the power first. Hold the power button until you get this display. Then slide to power off. Remove the SIM card by inserting a paper clip into the little hole on top of the phone.

Remove the 2 screws at the bottom of the iPhone.

The next step is to remove the broken glass. If your screen is cracked really badly, you can apply a piece of packing tape and then use the suction cup to pull off the glass. Start pulling from the bottm since there are ribbon cables at the top.

Remove the ribbon cables in the order they are labled 1,2,3. The last one is a little tricky. Flip the back of the connecter up as in the diagram below.

Once you get the cables disconnected. Remove the 6 screws and lift out the LCD screen, 2 will be under black tape. You can gently pry the outsides of the metal clips holding it in. The LCD should kind of pop out then you can slide it out toward the bottom.

Now the fun part! This is the most difficult step... Remove the glass by using a heat gun or blow dryer to soften up the glue. Be careful not to overheat it since you still have electronics on there. Start warming up one end and try to pry the glass from the plastic. It will take some time, so be patient.

Once the glass is removed, apply the 2 sided tape that should have come with your kit and apply the new digitizer very carefully. Make sure it is lined up perfectly.

Now replace the LCD, re-connect the ribbon cables and put the 2 screws at the bottom back in

If everything went ok, your iPhone should work just fine. If not check that the ribbon cables are connected back properly.