Double clutch - its stupid, you dont need it.
Double clutching helps preserve synchro life A LOT. It's a more advanced form of rev matching (which will also help your synchro life), mainly used when downshifting in the middle of a turn. It helps maintain the car's weight transfer by smoothly engaging the lower gear, instead of rev matching or just straight shifting into that gear. Otherwise, the car may lose its balance due to the excess weight transfer, which will either slow you down, or even make you lose control.
To double clutch, first push the clutch in, then move the shifter to neutral and let the clutch out. Rev the engine up to the RPMs that it will be at in the next gear, then push the clutch in, shift into that gear, and let out the clutch smoothly.