The steps are virtually the same on both, and I will mention where they differ. I will be working on a 16gb 3G[S] - lets get started.
Ok first thing - the supplies you will need:
- Small phillips screwdriver
- Suction cup w/ pull ring
- Case opening tool
I also recommend having a nice clean and clear work space, and to lay the screws and parts out as you pull them to keep them in order and to make sure you don't forget or lose anything when putting the phone back together.
Now to start work on the phone itself. First thing you will need to do is take out the two small screws at the bottom of the phone. They are on both sides of the dock connector.
Once both screws are taking out take your suction cup and stick it towards the bottom of the screen by the home button and lift hard enough to pull it up but not to hard to rip it right out of the phone. Now if you don't have a suction cup, no worries you can do the same thing pry putting your pry tool in between the glass and the bezel and prying up gently.
You will see that the screen is connected to the logic board there are 3 ribbons holding it in place. They are numbered 1, 2, and 3. The first two just pop right out, but ribbon 3 has a latch that will need to be lifted up and the ribbon will slide out. Now the screen is disconnected. If you just need to replace the screen stop here!
Now that you have the screen disconnected you have can have a look at the guts of the iPhone. (Not too much there for something that is so amazing huh?) Looking at it you will see 8 screws that you can see have a copper circle around them. One of them is hidden under the "do not remove" sticker - if you remove this sticker to get the screw, you have voided warranty. Take out the 8 screws. And pop the last of the numbered ribbons (the only major difference in the whole take apart is that the 3G only has 6 ribbons and 3G[S] has 7).
Once you have taken the screws out and popped the ribbons take the logic board out I find this easiest by pulling from the bottom and lifting up like illustrated below.
The battery is just glued into place - there is a pull tab to lift. I find that the tab is too weak for the glue so I take my pry tool or a butter knife and gently pry up on the battery til it pops off.
Next is the dock connector - these are very easy. There are 3 screws; one at the top and 2 down by the bezel just unscrew all 3 and lift it out.
Now for the hard part: the flex ribbon. There are 10 screws holding it all in place 2 for the power button, 1 for the headphone jack, 3 for the volume toggle, 2 for the mute toggle, and 2 to hold the ribbons in place. Take your time with these and don't get frustrated - they are a pain and at weird angles. Also, the screws are very small and short so try not to lose any.
Looks pretty empty now:
You're almost done! All that's left is taking the LCD off the digitizer. There are 6 screws holding them together; 1 at the top and 5 along the side there is tape covering them. Just peel that away to expose them.
Once all 6 are out what i like to do is at the bottom of the LCD is a little metal tab gently left up on it with you figure nail and just work the LCD out of the digitizer. Take your time not to crack the LCD - too much pressure will cause this.
And congrats! You have completely taken apart your 3G or 3G[S]. To put back together, read backwards.