Ladder #249 is five finishes in a row: 40, 60, 80, 99 and 127. You get one turn of three darts for each, and every finish has to end on a double or the bull.
40 is a straight double: D20. If the dart lands in the single by mistake you are left on 20, which still has a double finish with the darts you have left. 60 is a two-dart finish: 20 D20.
80 is a two-dart finish: T20 D10. 99 needs all three darts: T17 8 D20. If the first dart misses its treble, the board will show the new route straight away. 127 needs all three darts: T20 T17 D8. If the first dart misses its treble, the board will show the new route straight away.
At best the five finishes need 11 darts out of 15. Your result counts the finishes you make first and the darts you used second. The last finish is long. Throw the first dart at the big treble with intent, then let the board work out what is left.