The first thing you wanna do is head to the nether, there you will search a nether fortress (reminder: the nether is very scary so don't doodoo yourself). you will meet some tall black men, called wither skeletons, kill them and obtain their skulls (very low drop chance).
The next step is very simple, get 4 soul sand and make a plus formation with them.
place the 3 skulls on top of the blocks and there you have it!
you have succesfully spawned the wither in minecraft!