First, good hygiene practices. Clean your secators and the vase in whick you will put the flowers. Cut the stems with a straigth cut. Be aware not to crushed the stems.
Place them in a clean plastic pot containing warm water with flower food (Floralife or other). Be careful to eliminate leaves and debris from the water to maintain your flowers healthy minimising fungus disease in the water. Keep them away from a source of heat or from direct sun. And if your refriigerator is big enough keep them in cold storage during the night to extend the vase life of your peony flowers.