Pointers to archive or opinions welcomed.

thx

You should downshift on a regular basis. This, among other things, saves on brakes using the engine to slow down. Also, regular downshifting improves your ability to become one with the car.
And by doing it regularly you will be less likely to do it wrong in the future if needed. Avoid downshifting to first though. You will find in most cases to third is good enough. In the middle of the shift add rpm to synchonize the transmission to the engine making it smooth.

Never, never, pop the clutch when downshifting. Wrong gear could result in over revs and broken engine.

Start practicing at slow city speeds and increase the speeds over time. I can't imagine not downshifting.