Decide how hot you want the heater to get. This should be 10-20c below the lowest print temp you expect to run the bed at.

Test your resistor wire (nichrome in my case) with a 5v supply and a temperature probe. You want to find a length that uses as much current as possible without exceeding your lowest printing bed temp after 15 min of being powered (the heater may continue to heat up after 15 min, but hopefully not by much).

Once you have decided how long your resistor wires need to be. Its time to wind them onto bamboo rods so they can be held in place easily. Be careful to keep some space between turns as you dont want the wire to short to itself. This will reduce resistance, drawing more current and generating more heat. Kapton tape may be helpful here.

Secure your heating rods to the bottom of your heated bed or to the top of your sub bed. I drilled holes in my sub bed and used tie wire to secure the rods. Being careful not to short the 5v live heating element to anything conductive. Kapton tape may help here too. Don't bother trying to stick Kapton to wood. I tried and it does not work.

Once you have the heating rods secured used some wire capable of taking decent current to connect to the 5v rail of your ATX.

Print something to see how it works. Enjoy.