[Tom Saxton]

Typically, you'd ground your circuit with the ground pin on the parallel port. Then to set a data pin to zero, you'd connect that pin to the shared ground. To set it to 1, connect it to +5V relative to shared ground.

I haven't done much with parallel ports, but I'd also expect that you'd want to set the pin you're fiddling with to be an input, so the PC isn't also trying to set the pin value.

As an added safety, you probably want to connect the pin to ground or +5 through a resister so you limit the current that flows if something goes wrong.

Maybe this answered your question, or maybe you already know what I just said and are asking how to accomplish the connections. To answer that, I'd need to know what you're working with. For example, it's easy to do if you have a programmable microprocessor on your circuit board.