Grind a single-point tool bit to the shape of the thread form, including the root of the thread. This will ensure proper cutting of V-shaped threads with a 60-degree thread angle -- the most common type.
Position the bit so it is aligned perpendicular to the screw. Check your alignment with a center gauge.
Set the lathe's gear arrangements to the desired number of threads per inch using the lathe's quick-change gearbox.
Adjust your spindle speed and ensure the screw will turn in the proper direction for a right- or left-hand thread, depending on which you want. Set the compound rest to 29 degrees to the right for a right-hand screw or 29 degrees to the left for a left-hand screw. The compound rest feed handle will control the depth of the cut.
Consult engineering tables to ensure the proper major and minor diameters of your cut.
Place the bit tool against the right end of the screw and adjust the compound rest dial to zero. Adjust the carriage half-nut lever to connect the half nut to the screw. Use the thread chasing dial to engage the half nut at the appropriate time.
Cut your first thread by making a series of shallow cuts, each following the previously made groove. Your first cut should be extremely shallow, to allow you to check to ensure you have the proper pitch and lead. After each cut, move the bit away from the screw and reset the carriage so the bit is at the right end of the screw again.
Use a chanifer cutting bit to end the thread properly when the proper depth of cuts has been achieved.
Reset the lathe when the first thread is finished, then cut your second thread in the same fashion.