What is the proper technique for shifting gears in a commercial motor vehicle with a manual transmission (double clutching)?

1. Press the clutch, shift to neutral, release the clutch, press the clutch again, shift to the next gear, release the clutch
2. Hold the clutch down while moving through each gear
3. Shift without using the clutch at any time
4. Press the clutch once and move the shifter directly to the next gear