Making blocks directional (facing into certain directions) is also done using block states. This example describes a vertical version of the andesite slab.

public class VerticalSlabBlock extends HorizontalFacingBlock {
 
	public VerticalSlabBlock(Settings settings) {
		super(settings);
		setDefaultState(getDefaultState().with(Properties.HORIZONTAL_FACING, Direction.NORTH));
	}
 
	@Override
	protected void appendProperties(StateManager.Builder<Block, BlockState> builder) {
		builder.add(Properties.HORIZONTAL_FACING);
	}
 
	@Override
	public VoxelShape getOutlineShape(BlockState state, BlockView world, BlockPos pos, ShapeContext ctx) {
		Direction dir = state.get(FACING);
		switch(dir) {
			case NORTH:
				return VoxelShapes.cuboid(0.0f, 0.0f, 0.0f, 1.0f, 1.0f, 0.5f);
			case SOUTH:
				return VoxelShapes.cuboid(0.0f, 0.0f, 0.5f, 1.0f, 1.0f, 1.0f);
			case EAST:
				return VoxelShapes.cuboid(0.5f, 0.0f, 0.0f, 1.0f, 1.0f, 1.0f);
			case WEST:
				return VoxelShapes.cuboid(0.0f, 0.0f, 0.0f, 0.5f, 1.0f, 1.0f);
			default:
				return VoxelShapes.fullCube();
		}
	}
 
        @Override
	public BlockState getPlacementState(ItemPlacementContext ctx) {
		return super.getPlacementState(ctx).with(Properties.HORIZONTAL_FACING, ctx.getHorizontalPlayerFacing().getOpposite());
	}
 
}

public class ExampleMod implements ModInitializer {
    public static final VerticalSlabBlock POLISHED_ANDESITE_VERTICAL_SLAB = Registry.register(
        Registries.BLOCK,
        new Identifier("tutorial", "polished_andesite_vertical_slab"),
        new VerticalSlabBlock(FabricBlockSettings.copyOf(Blocks.POLISHED_ANDESITE)));
}

src/main/resources/assets/tutorial/blockstates/polished_andesite_vertical_slab.json

{
  "variants": {
    "facing=north": { "model": "tutorial:block/polished_andesite_vertical_slab", "uvlock": true },
    "facing=east":  { "model": "tutorial:block/polished_andesite_vertical_slab", "y":  90, "uvlock": true },
    "facing=south": { "model": "tutorial:block/polished_andesite_vertical_slab", "y": 180, "uvlock": true },
    "facing=west":  { "model": "tutorial:block/polished_andesite_vertical_slab", "y": 270, "uvlock": true }
  }
}

src/main/resources/assets/tutorial/models/block/vertical_slab.json

{   "parent": "block/block",
    "textures": {
        "particle": "#side"
    },
    "elements": [
        {   "from": [ 0, 0, 0 ],
            "to": [  16, 16, 8 ],
            "faces": {
                "down":  { "texture": "#bottom", "cullface": "down" },
                "up":    { "texture": "#top",    "cullface": "up" },
                "north": { "texture": "#side",   "cullface": "north" },
                "south": { "texture": "#side",   "cullface": "south" },
                "west":  { "texture": "#side",   "cullface": "west" },
                "east":  { "texture": "#side",   "cullface": "east" }
            }
        }
    ]
}

src/main/resources/assets/tutorial/models/block/polished_andesite_vertical_slab.json

{
    "parent": "tutorial:block/vertical_slab",
    "textures": {
        "bottom": "block/polished_andesite",
        "top": "block/polished_andesite",
        "side": "block/polished_andesite"
    }
}

For directional blocks, you may have to override rotate and mirror methods, so that in structure blocks, they can be correctly rotated or mirrored. However, in this case, the HorizontalFacingBlock class has already done it for you.

Try to make it waterloggable.