It is best to take stem cuttings with growing roots. You will see your string of bananas grow some roots even though they're not in the soil. These cuttings are about a foot long.
I suggest you use cactus mix to plant them in. I'm using regular potting Soil for my String of bananas.
Stick them in the pot (hanging pots) about half an inch deep.
Water well and place in partial shade. In a few weeks it will start to root and show signs of new growth.
