There's a couple of ways:
1. Do what everyone else does, kick the clutch and shift to the lower gear, ease the clutch back out so the engine matches your vehicle speed.
2. Double clutch by kicking the clutch, shift to neutral, release the clutch, rev it up to what you think the next gear will engage at, kick the clutch again and pop it into the lower gear, release the clutch and open the throttle ... and you got a smoother downshift than normal downshifting.
Double clutching requires practice, and if done right, you will not feel any jerkyness during the procedure; it should take no more than a fraction of a second to perform. At certain engine speeds (RPM), you can shift without using the clutch at all, so rev it up high, let it fall down and if you got the ballz, apply pressure on the stick towards the gear you want ... it should fall right into it at the right RPM.
Normal downshifting will wear your clutch and synchros, while double clutching by rev matching will save them. If you prefer to save your brakes by engine braking, double clutching is your best solution coz brakes are cheaper than clutches and synchros. Think about that.....