The process is the same as caring for tulips:
Leave the protective sleeve on and trim the bottom of them stems overnight
Place the bouquet with the sleeve on in a vase filled 1/3 with water.
The next day remove the sleeve and arrange your flowers. Then enjoy!
Make sure you keep an eye on the water level and keep refilling the container.  Peonies absorb water faster than other flowers. It's best if you keep the blooms away from heat and moisture. Keep it in a cool and dry place to keep them fresh and healthy for long.