Galvanised pipe needs threads on the ends to screw into fittings. Standard lengths of pipe may come already threaded, but if you cut pipe to a custom length, you'll need to thread the ends yourself. Inexpensive rethreading dies repair damaged threads but if you're starting with plain pipe, look for dies specifically designed for cutting new threads. For a one-time job, rent a thread-cutting die, handle and pipe vice to avoid investing in equipment you won't use often.
Place the pipe in a vice with the end that you want to thread extending about 2 to 3 inches beyond the vice. Tighten the vice securely to keep the pipe from rotating.
Coat the area where you want to cut the threads with cutting oil, or regular lubricating oil if you don't have any cutting oil.
Look at the numbers marked on the face of the dies, if you have more than one, and choose a die marked the same as the diameter of the pipe. The cylinder-shaped die has an opening the size of the pipe containing cutting teeth.
Insert the die into the ratchet or handle that holds the die and gives you leverage to turn it.
Slip the die onto the end of the pipe and if your handle has a ratchet, set it to the "cut" position, so you can apply force clockwise.
Turn the handle clockwise while pressing the die against the end of the pipe with your other hand to get it started. Once you feel resistance, turn the handle twice, stop and add more oil to the pipe and die. Turn the handle twice more, stop and oil again. Repeat until you've cut threads as far as you need.
Switch the ratchet to the reverse position if necessary and turn the handle counter-clockwise to unscrew the die from the pipe.
Use plenty of oil or the pipe may swell from friction-caused heat and jam in the die or damage it.