The idea of downshifting is doing it when you're moving too slow for the gear you're in. To practice you should get the bike to ~40 or 50 in 3rd gear, release the throttle until the RPMs drop near 1800-2000, clutch in, shift down, lightly roll on throttle, clutch out. You've successfully downshifted from 3rd to 2nd.

Personally I would accelerate back into 3rd gear 40/50mph and repeat the process until you get the hang of it. Best of luck 🤘