AWS autoscaling - custom scale in process

I have a need to customize scale in event. Specifically I have to trigger scale in event in a controlled way - decrease desired size by 1 and terminate one specific instance. This will be done from AWS SDK.

What is the correct order to issue commands: A) terminate instance, decrease desired size. B) decrease desired size, terminate instance.

I want to avoid situation where Autoscalling will terminate another instance when desired size parameter changes.