I've seen projects similar to this but the trigger mechanisms didn't seem too reliable. So this project is how to install a pressure switch to make your Altoids tin musical. If you haven't tried soldering before this project should be a good start. It only has four simple solder joints to make.
Step 1: Everything you need.
For this project you will need
Musical greeting card.
Micro switch (radioshack $1.69)
18 or 20 gauge solid core wire
Spool of Solder
Solder station with helping hands.