My wife's iPhone 3GS fell into a concrete floor few days before the 4s release, coincidence?.
It was hard at first time, but the videos help a lot. I did not install properly one of the connectors and I had to unmount the iPhone again.
I bought the full front panel to avoid dissasembling just the touch screen so the fix was not that hard.
Be calm, think it twice and watch the ifixit video tutorial.