when you are travelling at a high speed, and you want to shift to a lower gear, you want to make sure you do so at a speed which would not have the engine above redline in the target lower gear.
for example, you are travelling at 120kph in 3rd or 4th gears, and you slow down for a corner, planning to downshift to 2nd gear. This means that you have to be below (about) 97kph to be able to safely downshift to 2nd gear (i.e., the "II" marking on your speedometer). Presumably, you will be braking for the turn, ready to downshift to 2nd, and you should wait until the car slows down to this speed before engaging 2nd gear.
for best results, you should be rev-matching, and if you can, heel-and-toe downshifting. This is for a manual transmission, of course.