Well, the simplest way is to get a hotshoe adaptor with a sync terminal. You would mount your flash on the hotshoe adaptor and connecting the cable to the sync terminal of the hotshoe adaptor.
In the above picture, this hotshoe adaptor not only has a sync terminal, it also has an optical slave. This device cost about $20. The optical slave is also useful, and it is the cheap alternative to the Nikon SU-4 device (which cost about $180). When the optical slave optically detects that a flash has been fired, it would then trigger the flash mounted on the adaptor. With this device, you can also trigger your flash wirelessly, but what you need is line of sight between the flash that is being fired and the optical slave. In bright outdoors environment, the optical slave may not work very well.