The process is very similar to replacing a cracked screen. I broke my iPhone 4 a while back and spent quite a lot of time reading up on this fix.
The biggest issue you will encounter is the quality of the new display, as you need a white screen to change colours. From what I gathered quite a lot of these websites don't sell legit apple components so the risk of getting a screen that might not be as awesome as the original is quite real. If the people you buy from offer to replace the component you buy should you not be happy with the quality the rest of this process is a breeze.
Make sure to spend the time on YouTube to familiarize yourself with the phone and the process and be sure to download an A3 component diagram as this allows you to keep track of what you have taken out and where it needs to go when you reassemble. For a first timer this mod can be quite a slow process so make sure you have ample time available. Just be sure to take a look at the button as for my fix there was no need to replace it, not sure how it is attached to the motherboard but you should be careful when removing it.
Besides for the above, it's really not rocket science, and not worth the price people charge. Take a look at icrackedit.co.za, they offer very good service.
