while not turtle.detectDown() do
  turtle.down() -- get the turtle down to the ground.
end
turtle.up() -- to keep the turtle 1 block up in the air.

-- I got 2 pieces of log, and I want to craft a chest...
-- The log is in the first position in the turtles inventory
turtle.select(1)
turtle.craft(2)

for i = 1, 9 do -- I THINK this will execute 8 times, starting on 1...
  if i == 4 or i == 6 or i == 8 then i = i + 1 end -- to get the crafting grid for a chest.
  if i ~= 8 then turtle.dropDown(8 - i) end -- drop the stuff below the turtle, unless it's on it's 8th run.
  if i ~= 1 then turtle.select(i) end -- select the correct slot (not on first run)
  if i ~= 8 then turtle.suckDown() end -- pick everything thats below up again, putting it in the expected slot.
end

if not turtle.craft() then
  print("Failed crafting!")
end

-- so the turtle should place wooden planks in this pattern:
-- [ ] [ ] [ ]
-- [ ] N/A [ ]
-- [ ] [ ] [ ]
-- unless someone else picks up the blocks or if extra blocks where placed below