Forge Block State for Stairs, using only one file.

ForgeStairs.json

{
    "forge_marker": 1,
    "defaults": {
        "model":"stairs",
        "textures": {
	        "bottom":"myMod:blocks/texture",
	        "top":"myMod:blocks/texture",        
	        "side":"myMod:blocks/texture"
        },
        "transform":"forge:default-block",
        "uvlock": true
    },
    "variants": { 
        "inventory": [{
            "transform": {
                "gui": {"rotation": { "y": 180 }}     
           }
        }],
        "facing":{
            "east":{},
            "west":{"y": 180},
            "south":{"y": 90},
            "north":{"y": 270}                        
        },
        "half":{
          "bottom":{},
          "top":{"x": 180}  
        },    
        "shape":{
            "straight":{},
            "outer_right":{"model":"outer_stairs"},
            "outer_left":{"model":"outer_stairs"},
            "inner_right":{"model":"inner_stairs"},
            "inner_left":{"model":"inner_stairs"}
        },         
        "facing=east,half=bottom,shape=outer_left":  {"y": 270},
        "facing=west,half=bottom,shape=outer_left":  {"y": 90},
        "facing=south,half=bottom,shape=outer_left": {"y": 0},
        "facing=north,half=bottom,shape=outer_left": {"y": 180},
        "facing=east,half=bottom,shape=inner_left":  {"y": 270},
        "facing=west,half=bottom,shape=inner_left":  {"y": 90},
        "facing=south,half=bottom,shape=inner_left": {"y": 0},
        "facing=north,half=bottom,shape=inner_left": {"y": 180},
        "facing=west,half=top,shape=straight":  {"x": 180, "y": 180},
        "facing=south,half=top,shape=straight": {"x": 180, "y": 90},
        "facing=north,half=top,shape=straight": {"x": 180, "y": 270},
        "facing=west,half=top,shape=outer_right":  {"x": 180, "y": 180},
        "facing=south,half=top,shape=outer_right": {"x": 180, "y": 90},
        "facing=north,half=top,shape=outer_right": {"x": 180, "y": 270},
        "facing=east,half=top,shape=outer_left":  {"x": 180, "y": 90},
        "facing=west,half=top,shape=outer_left":  {"x": 180, "y": 270},
        "facing=south,half=top,shape=outer_left": {"x": 180, "y": 180},
        "facing=north,half=top,shape=outer_left": {"x": 180, "y": 0},
        "facing=east,half=top,shape=inner_right":  {"x": 180, "y": 0},
        "facing=west,half=top,shape=inner_right":  {"x": 180, "y": 180},
        "facing=south,half=top,shape=inner_right": {"x": 180, "y": 90},
        "facing=north,half=top,shape=inner_right": {"x": 180, "y": 270},
        "facing=east,half=top,shape=inner_left":  {"x": 180, "y": 90},
        "facing=west,half=top,shape=inner_left":  {"x": 180, "y": 270},
        "facing=south,half=top,shape=inner_left": {"x": 180, "y": 180},
        "facing=north,half=top,shape=inner_left": {"x": 180, "y": 0}
    }
}

NewStairs.json

 {
   "forge_marker": 1,
   "parent": "forge_stairs",
    "defaults": {
        "textures": {
           "bottom":"myMod:blocks/texture", 
           "top":"myMod:blocks/texture",         
           "side":"myMod:blocks/texture" 
        }
    }
}