I've created this little shutter release switch a while ago, but decided to create a small step by step with pictures.
You can use this to even control two cameras at a time! Just make a second cable and there you go, control two cameras :D

Step 1: Well, collect all the parts needed

Used parts:

*One RJ11 phone line cable
*one switch, two push buttons
*one phone line RJ11/RJ12 splitter
*one 2.5 mm TRRS connector
*one CANON EOS 450D
*soldering equipment
*cutter and the usual other tools