The best easiest way i can explain it is just listing the block type of the stairs and where the hitbox it is using.

Stairs(Facing, half, shape)-----------Hitbox(Hitbox being used)

North, bottom, strait---South, bottom, strait
South, bottom, strait---East, bottom, strait
East, bottom, strait---North, bottom, strait

North, bottom, inner_right---South, bottom, strait
North, bottom, inner_left---South, bottom, strait
South, bottom, inner_right---East, bottom, strait
South, bottom, inner_left---East, bottom, strait
East, bottom, inner_right---North, bottom, strait
East, bottom, inner_left---North, bottom, strait
West, bottom, inner_right---West, bottom, strait
West, bottom, inner_left---West, bottom, strait

North, bottom,outer_right---South, bottom, strait
North, bottom,outer_left---South, bottom, strait
South, bottom,outer_right---East, bottom, strait
South, bottom,outer_left---East, bottom, strait
East, bottom,outer_right---North, bottom, strait
East, bottom,outer_left---North, bottom, strait
West, bottom,outer_right---West, bottom, strait
West, bottom,outer_left---West, bottom, strait

North, top, strait---South, top, strait
South, top, strait---East, top, strait
East, top, strait---North, top, strait

North, top, inner_right---South, top, strait
North, top, inner_left---South, top, strait
South, top, inner_right---East, top, strait
South, top, inner_left---East, top, strait
East, top, inner_right---North, top, strait
East, top, inner_left---North, top, strait
West, top, inner_right---West, top, strait
West, top, inner_left---West, top, strait

North, top, outer_right---South, top, strait
North, top, outer_left---South, top, strait
South, top, outer_right---East, top, strait
South, top, outer_left---East, top, strait
East, top, outer_right---North, top, strait
East, top, outer_left---North, top, strait
West, top, outer_right---West, top, strait
West, top, outer_left---West, top, strait