Checkout ladder #253 climbs from 4 to 108 in five steps — 4, 55, 70, 98 and 108 — with three darts per finish and a double to end on.
4 is a straight double: D2. If the dart lands in the single by mistake you are left on 2, which still has a double finish with the darts you have left. 55 is a two-dart finish: 15 D20.
70 is a two-dart finish: T18 D8. 98 is a two-dart finish: T20 D19. 108 needs all three darts: T17 17 D20. 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 middle finishes decide this ladder. Choose the route that leaves a double after a missed treble; the suggested routes do that wherever they can.