The reason I made this guide is because I'm tired of people saying the Wither is a gear check and that there's no strategy involved in fighting it.
First thing you need is to make a big flat area out in the open, around 50x50 made out of a block with decent blast resistance, I find cobblestone works good and is easy to get. Don't make it out of dirt or sand otherwise he can easily create holes for you to fall into while your moving around.
Next you need to get you weapons and potions ready what you'll need is
1 Power IV or higher bow with knockback II
1 Diamond sword with smite IV
1 Extended Potion of Regeneration
1 Extended Potion of Speed
1 Extended Potion of Strength
3 Splash Potions of Instant Health II
3 Wither Skulls
4 Soul Sand
Next make sure that it's early in the day and that there are no animals or mobs anywhere nearby.
After doing that set up the Wither like this and get all of you items on the hot-bar however you want them.
STRATEGY FOR THE FIGHT
Some thing you should know about the Wither's skulls it launches out, they only apply the wither effect if they hit you directly and they have a delay before being fired again. As long as you don't take a direct hit the explosions cause by the skulls won't do to much damage to you.
First thing you should do after placing the last head on it is to drink your extended potions while it's filling up the health bar, back away from the Wither while drinking them, also drink your regeneration potion last.
After you're far enough away from the Wither so that its explosion won't hit you pull out your bow and aim a bit above it so that when it launches into the air you can get a quick shot on it.
Now comes the hard part you have to avoid getting hit by the skulls while still shooting the Wither. First up you should never stop moving, if you stand still you will die. Just keep backing up while shooting. The best way to do this is to wait for the Wither to fire its skulls and shoot it between the delay. If you think you're going to get hit jump right before the skull hits you so that it goes under you and you only take explosion damage. Even without the wither effect this could end up doing around 4 full hearts of damage.
Never let the Wither get directly above you, if it does sprint away from it or use your knockback bow to launch it away. If the Wither gets above you it becomes almost impossible to dodge the skulls.
After it's down to half health get your sword ready. You'll want to get close to the Wither fast because it will still shoot skulls at you. When you attacking the Wither circle around it while attacking so that skulls are less likely to hit you. With any luck the Wither will die and you have a brand new nether star.
Hope you enjoyed this guide an if you have any suggestions for improvements feel free to tell me.