Checkout ladder #256 climbs from 12 to 122 in five steps — 12, 51, 65, 89 and 122 — with three darts per finish and a double to end on.
12 is a straight double: D6. If the dart lands in the single by mistake you are left on 6, which still has a double finish with the darts you have left. 51 is a two-dart finish: 11 D20.
65 is a two-dart finish: T19 D4. 89 is a two-dart finish: T19 D16. 122 needs all three darts: T17 T17 D10. If the first dart misses its treble, the board will show the new route straight away.
At best the five finishes need 10 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.