Minecraft wiki entry for Achievement_guide, associated text: To increase spawn rates, break down the walls to have more floor space
Remove any blazes and their spawners, (unless you are skilled in making experience farms using them.)
Wear diamond armor (or netherite) and bring a fire resistance potion (for blazes and accidental lava deaths).
It is also helpful to have a diamond sword (or netherite) with the looting III enchantment.
Have lots of patience. You'll need it since wither skulls are a rare drop and you need three.
Be careful of the wither skeleton's attacks. They give you the wither effect, which is poisonous and extremely deadly. It may be a good idea to bring milk.
Snowballs are also a terrific way to defeat blazes.
Every 15 blocks or so, put a bar so the walkway is only two blocks high. Wither skeletons cannot walk under anything that is less than three blocks tall.