Wither

local sand = {}
local skull = {}

for i = 1, 16 do
  local t = turtle.getItemDetail(i)
  if t then
    if t.name == "minecraft:soul_sand" then
      sand[i] = t.count
    elseif t.name == "minecraft:skull" then
      skull[i] = t.count
    end
  end
end

local function place()
  while not turtle.place() do sleep(0.05) end
end

local skullSelected, sandSelected
local function placeSkull()
  if not skullSelected or skull[skullSelected] == 0 then
    if skullSelected then
      skull[skullSelected] = nil
    end
    skullSelected = nil
    for pos, c in pairs(skull) do
      if c > 0 then
        skullSelected = pos
      end
    end
  end

  if not skullSelected then
    print("out of skulls")
  else
    turtle.select(skullSelected)
    place()
    skull[skullSelected] = skull[skullSelected] - 1
  end
end

local function placeSand()
  if not sandSelected or sand[sandSelected] == 0 then
    if sandSelected then
      sand[sandSelected] = nil
    end
    sandSelected = nil
    for pos, c in pairs(sand) do
      if c > 0 then
        sandSelected = pos
      end
    end
  end

  if not sandSelected then
    print("out of sand")
  else
    turtle.select(sandSelected)
    place()
    sand[sandSelected] = sand[sandSelected] - 1
  end
end

local function up()
  while not turtle.up() do sleep(0.05) end
end

local function down()
  while not turtle.down() do sleep(0.05) end
end

local function back()
  while not turtle.back() do sleep(0.05) end
end

local function forward()
  while not turtle.forward() do sleep(0.05) end
end

placeSand()
up()
forward()
placeSand()
back()
placeSand()
back()
placeSand()
up()
forward()
forward()
placeSkull()
back()
placeSkull()
back()
placeSkull()
down()
down()
forward()