Tip on getting pin out
I found that I was able to get the screen retainer pin out by taking the smallest flat-blade screwdriver I have (with a tip like 1mm wide) and gently inserting it behind the pin (toward the outside edge of the phone). This made it possible for the head of the pin to slide out. When I had done this, the pin slid out very easily. So, I'd say if the pin isn't sliding smoothly, you don't have it aligned right.
Also, take pictures of each step in case you forget what piece goes where. Particularly important is to take close-up pictures of the screws that hold the battery cover in place. The phillips head screws are shorter and there are only 3 of them. If you don't remember the places they go into (one in one corner, the other two in the opposite corner), you'll waste a lot of time trying to figure it out.
And, as noted elsewhere, this will void your warranty. I read somewhere that just removing the little black sticker over the retaining pin is what alerts Nokia that the phone's been fiddled with.
Good luck.