In order to make a wither, first you need to go into the nether.
Now find some soul sand, collect at least four of them.
After that, you need to find a nether fortress.
Nether fortresses have wither skeletons spawners, you can kill them to get wither skulls. Wither skulls are rare drops so you need to be patient at this stage to get them.
Once you have three wither skulls, you need to go to the place where you want to build the wither.
Go out of the nether and come back to the biome to choose the place. And place four soul sand blocks in a T shape.
Then place three wither skulls over these soul sands. You will get to see a wither right after placing the third wither skull.