This will definitely VOID YOUR WARRANTY so don't say you haven't been warned.
I suggest you only use this guide if you were desperate like me. My phone didn't come with an official warranty so I was as good as screwed if I wanted a new screen.
Step 1: Obtain the part
Just ebay for lcd screen and digitizer galaxy s2 (however u may wish to buy it with the faceplate see below note). You want both the super amoled plus screen and the digitizer together. You can get this for about 120-130 USD.
There are some vendors that sell just the 'lens' but you need a specialized machine to replace it and is simply not worth the trouble.
Note that if you go to an original Samsung service centre, when they order the part, it comes together with the face plate and is therefore a breeze to install. This part is for sale on Ebay too. http://cgi.ebay.com/LCD-Touch-Digiti...item3a682b9ad0. I would recommend you to buy this type instead of just the LCD and digitizer.
Follow this disassembly guide (not mine).
Now using a thin metal flat tool with a wide flat surface area, dig at the small gap from the top of the screen and pry the screen from the frontplate. It is held entirely by double-tape so don't worry about any screws, catches. Be careful of not damaging the bezel in the process.
Clean off the old black tape and double tape as best as u can, and be careful of the bottom buttons. The attachment shows how it looks like after cleaning it.
Reapply the double tape in strips and try to shape it to fit as much as you can. This is tiresome but necessary since the adhesive behind the lcd screen is very weak and you don't your screen falling out of you halfway.
Push it in and it should stick. That's it! Then reassemble.