Buy the relevant size plugs for the device you want to connect into, a standard 1/4 inch line jack socket, an aircraft mike line jack socket, and a length of twin screened cable (or twice the length of single-screened).
Take your trusty soldering iron, and connect the mike plug and mike socket to the ends of one line of the screened, and the headphone plug and headphone socket to the other.
All except the aircraft mike line socket are easy to obtain.
Since the microphone in your headset won't work with the computer, stereo, etc, I'd just do the earpiece bit. You can buy adaptors to connect a 1/4 inch jack to a 2.5mm or 3.5mm input - I reckon on paying about £2 for one.
If your headset is a standard aviation one, it won't be stereo, so you will need to make up a lead that connects the two outputs of the stereo end to the one input of the headset. I think you can buy adaptors for that too, but I've never tried.