Watch tool tutorial
1: On the inside of the bracelet/strap you will find small arrows on each removable link which indicate which way the pins should be pushed out.
2: Then place the bracelet/strap in the tool so that the pin can be pushed out the right way. (Make sure to hit the hole/pin precisely, otherwise you may break/bend the pin on the tool.)
3: When the desired number of joints has been removed, insert the pin in the same way as you removed it. If the pin still sticks out a little after this, you can press/hammer it all the way in with the base of the tool.