First, you need your tools. You can use a Phillips head screwdriver and a socket wrench, or a skate tool. I prefer using a skate tool, because they have all the socket sizes you will need, and tend to be pretty cool. Throughout these instruction photos, I will be using a Project brand skateboarding tool.
Make sure that the skate tool you are using has a detachable Phillips head screwdriver, or you will need to get one of your own. Also, make sure that the hardware that you use actually uses Phillips heads screws. Some hardware uses hex head screws, and some don't use screws at all. Take a look at your skateboard's hardware, and see what you need.
Also, throughout these instruction photos, you will notice that we removed the wheels first. You don't need to do that if you don't feel like it - we wanted to wheels out of the way for the photos, so you can see what we did.
And now, let's get started with taking those trucks off!