How to Spawn a Mob
While playing the game press the forward-slash key (/) and a small window will pop up. Enter the code, press enter and this will be enough to have your command activated.
/summon <entity_name> [x] [y] [z]
Brings a mob to a designated location. Get rid of the “[x] [y] [z]” at the end and you can have the mob spawn on top of you if you wish. Be careful though – if you’re spawning the Wither have to be quick to get away to save your skin.