Install a Wither Storm mod or add-on for your version of Minecraft
Open Minecraft with the mod or add-on enabled
Enter Creative mode
Obtain the Wither Storm spawn item or command from the mod
Place the required blocks or use the required summon command exactly as the mod specifies
Follow the mod’s spawning steps, since the Wither Storm is not in vanilla Minecraft
If using commands, enable cheats in the world
If using a modpack, check its documentation for the exact spawn method