A properly rev-matched downshift is seamless and causes zero wear on the transmission's synchos or the clutch.
And figuring out how to match isn't rocket science. For example if you find yourself wanting to shift into 3rd at 55mph, accelerate up into third gear from a stop and see how many RPMs your motor is turning in order to hold that speed. Let's say it's 5800rpm.
Later, when you're cruising in 6th gear and wish to drop to 3rd, clutch and shift to neutral. Blip the throttle til you catch the needle just under "6" on the tach with a glance, slip the trans
effortlessly into 3rd, and release the clutch quickly so the revs don't drop before you do it. The key is to rev-match before pulling the shifter into 3rd. Most semi-skilled drivers will do it after shifting into 3rd, before releasing the clutch pedal. This keep you from dragging the flywheel up to speed by burning your clutch disc against it, but all the "rev matching" inside the transmission is still being done with the synchronizers.
When you get that down, learn to heel and toe a downshift in a corner: Apply the brakes approaching a corner with the toe of your foot as normal, use the side of your foot or the heel to smoothly revmatch and downshift the transmission DURING the brake zone, so that as soon as you enter the corner you're in your power band in a lower gear and ready to accelerate through the apex and out the exit. You're holding the clutch down with your left foot nand modulating both of the other pedals with one foot at the same time.