What makes a Wither Skeleton spawn?

They naturally spawn in Nether Fortresses, light level 7 or less where their lower half will be.

To increase the chances of one spawning, make sure the light level condition is met, and fill any holes in the floor. It doesn't matter about material, wither skeletons will spawn whether there is nether brick or not.

In nether fortresses, skeleton eggs/spawners will spawn Wither skeletons 80% of the time.

According to the wiki:

"Wither skeletons only naturally spawn in nether fortresses. Brick type does not matter with naturally spawned Withers (e.g., replacing a Nether Fortress' bricks with normal stone bricks will still allow wither skeletons to spawn); they follow the same spawning conditions as naturally spawning Blazes: light level 11 or less where their lower half will be (needs confirmation).

When used in the Nether, Skeleton spawn eggs are able to spawn Wither Skeletons instead of the normal skeletons 80% of the time. Similarly, when a spider is spawned in the Nether (whether via spawn egg or a mob spawner), it has a slight chance of spawning with a wither skeleton jockey instead of an ordinary skeleton."

Based on that information you can use eggs or a mob spawner to get them to show up in the Nether.