You can down shift in an emergency or on downhill roads to keep from hard braking on a possible wipe out slippery area for more stable extra stopping power depending on the specific vehicle but you should always instinctively go for the (proper) gear you anticipate you will need, say you are slowing for a cloverleaf curve and pick 3rd gear because that's the best gear to be in for you speed so prepare to hit that gear as soon as you start to slow down. Don't try downshifting a gasoline engine until you know how risky it is or if needing a lower gear downhill. Gas engines don't have much braking power so one must be a very clutch worthy driver to properly downshift without taking the chance of floating the engine valves. When anticipating the gears properly, it will become second nature. I can smoke a cig, drink my coffee, and carry on a conversation on the CB radio while driving a manual transmission. That skill only comes from many years of driving and proper timing. When I drive a clutch, I do it so effortlessly that I don't even know I'm doing it, like it's burned into my reptilian part of my brain. Some people never get it and others take to it like a baby blanket, it all depends on the drivers skill and aptitude to adjust. I can drive a commercial truck tractor with a 2 speed axle so I may not be the best person to put things into the written layman perspective, but I could easily teach it to a driver from the passenger seat. The longer you drive a manual clutch, the more it becomes second nature. When I drive automatic cars and get into a panic brake, I stomp for the missing clutch pedal instinctively because of my many years of manual transmission vehicle driving experience.
Best of luck to you and your vehicles clutch! Practice makes you better, more practice with a on par instructor would be good.