to spawn the wither boss you need soul sand and wither skeleton skulls first you select soul sand in building blocks in your inventory then you put one block of soul sand down then you put another on on top then you put two soul sand blocks on the sides to make it look like arms then you select wither skeleton skulls from the decoration blocks in your inventory then you put three wither skeleton skulls on top in a straight line 

reminder
This only works on creative mode