Tutorial authors know that very often would be useful to have a third hand to be able to take pictures while hands are busy. As a workaround I created for my Nikon D5100 digital SLR camera a foot pedal.
I bought a taiwanese remote cable since the original Nikon MC-DC2 is too expensive to be “sacrificed”. You can see that inside there are three metal strips.
NOTE: I don’t know if keeping the focus and shutter wires connected could damage the camera. So I take no responsibility if you decide to imitate my project.
You can find some very useful tips for other cameras here.