Modern off-camera flashes aren't cheap. If you'd like to mess around with flash photography on the cheap, this handy tutorial shows you how to turn a stereo cord and some spare parts into a flash cord for vintage flashes.
If you've tried to use a vintage off-camera flash you've likely been frustrated by misfires and poor exposures which result from combining them and modern on-camera flashes. Red eye and TTL-metering flashes from modern cameras trick the slave sensors on the vintage flashes which causes them to fire too soon and underexpose your image.
DIY blog Highly Entropic solved this problem by using a simple stereo cord and a homemade hot-shoe adaptor made from a scrap of PVC plastic to tether the vintage flash to a modern camera. In doing so they were able to skip using the pop-up flash as a trigger and send the flash signal directly to the off-camera vintage flash. Check out the full guide at the link below for more information.
Don't Throw Away That Old Flash Yet! [Highly Entropic]