How did God tell Gideon to select his soldiers?

He told Gideon to select the soldiers who lapped water like a dog.

And the LORD said unto Gideon, The people are yet too many; bring them down unto the water, and I will try them for thee there.

So he brought down the people unto the water: and the LORD said unto Gideon, Every one that lappeth of the water with his tongue, as a dog lappeth, him shalt thou set by himself; likewise every one that boweth down upon his knees to drink.

And the number of them that lapped, putting their hand to their mouth, were three hundred men: but all the rest of the people bowed down upon their knees to drink water.

And the LORD said unto Gideon, By the three hundred men that lapped will I save you, and deliver the Midianites into thine hand: and let all the other people go every man unto his place. Judges 7:2-7