I have a mid-2009 Macbook Pro which originally had 4 GB of RAM. I wanted to bring it up to 8 GB for improved performance.
The upgrade went very smoothly. The tools I bought from iFixit were exactly what I needed and I just followed the online guide. When I booted up the Macbook afterwards, it recognized the 8 GB of RAM with no problem. I have since monitored my memory usage with Activity Monitor and the extra memory is being used. By the way, I have never added memory to a computer before.
Besides looking at the online guide for the memory upgrade, I also looked at the guide for changing the battery since I would be removing the battery during the process. I'm glad I did because I saw that I would need a Spudger for removing the battery. I also used a small towel to lay the small screws on so that I wouldn't lose them.