I use an automotive hose clamp as a guide and cut with a 32 tpi hacksaw. Then I square up the cut end with a flat file. The top of the steer tube needs to be about 1/8 inch (3 mm) below the top of the stem when fully assembled.
I recommend using a compression plug that offers the most support on the backside of the steer tube. Set the headset pre-load with the cap bolt. Set the pinch bolt torque with a 1/4 drive torque wrench with no more than 44 INCH pounds or 5 nm of torque. Don't use torque specs shown on the stem.