One day I turned on my Macbook and the battery icon had an "X" on it. When I unplugged the charger, my MB turned off. For a couple of weeks, I had no portability, because I had to have the charger in. Then all of a sudden, it just wouldn't turn on at all. I did a google search on my iPhone and discovered this wonderful place called ifixit.com. I ordered the part and had it the next business day with next day shipping.
Before I even got the part, I had my MB opened up and ready for the new battery to be plugged in, screwed up, and ready to turn on. I did just that, but then, a problem arose. My OS was corrupt or something or other. I was stuck at the gray Apple screen for hours. So I just put in my OSX install disk and did a CMD+R boot, and just reinstalled my original OS. All is well and I am back on the wonderful world wide web! God, I love the internet.
Putting a battery in is by far the easiest thing to do. The hardest thing would be finding small enough driver heads to unscrew screws, but then again that's not even that hard either.