Put the bike on the kick stand.
Push the rear, S-shaped derailleur up, to put slack in the chain.
Slide the chain back onto the rear cassette on the sprocket that corresponds to the gear that you were in when it came off. The rear sprockets that are larger hold the lower gears, and the smaller rear sprockets accommodate the higher gears. The front sprockets are the reverse of this, with higher gears on the larger sprockets and lower gears on the smaller ones. (See References 1)
Release the rear derailleur and let it slip back into place.
Apply WD-40 to the chain to make sure it functions smoothly. Most people do not carry WD-40 around in their back pocket; so if you don’t have any available, do this as soon as you get your Huffy home. Your chain should be in good condition for optimal functioning. If your chain is dry or rusty, it will be stiff and will not shift well. (See References 2)