By adding –s if there is a vowel before the -y

Singular	Plural		Singular	Plural
alloy	alloys		joy	joys
bay	bays		key	keys
boy	boys		kidney	kidneys
chimney	chimneys		monkey	monkeys
day	days		play	plays
decoy	decoys		ray	rays
delay	delays		toy	toys
donkey	donkeys		tray	trays
guy	guys		trolley	trolleys
jersey	jerseys		valley	valleys
journey	journeys		way	ways