Finishing in darts
Scoring gets you close. Finishing is a separate skill, and it starts a turn before you reach a double.
Pick a home double
Double 16 is the most popular finish for a reason: miss it into the single 16 and you are left on 16, which is double 8; miss that into the single and you are on 8, double 4. Double 20 and double 8 halve just as well. Odd doubles do not: a single 7 from double 7 leaves 7, which needs two more darts.
Leave a finish, not a score
When your score drops under about 200, stop thinking about the biggest treble and start thinking about the number you leave. On 210, a treble 20 and two singles might leave you on 130 — a finish. Two trebles might leave 90 — also a finish. Throwing the third dart at the treble when it would leave 159 or 162 is a waste: those have no three-dart finish.
When the first dart misses
Most three-dart finishes start with a treble. If it lands in the single, check what is left before throwing again. Often there is still a two-dart route; if not, use the second dart to leave a double for the third, or for the next turn.
The numbers that cannot be finished
With three darts, 169, 168, 166, 165, 163, 162 and 159 cannot be checked out. Above 110, nothing finishes in two darts, and only doubles and the bull finish with one. The checkout chart lists a route for every other number, and each finish has its own page with alternatives.