Relevance of Ear Training for Composers
Even though it might not seem necessary directly for composition, ear training is one of the most important things for a quick composition workflow. You can’t be bothered struggling to find that starting note of the melody that you hear in your head on top of these chords that you just play on the piano when you’re just in the middle of composing. By the time you have found it on the piano, the idea might just have vanished already.
There are several (even free) training programs online but one thing that doesn’t only help you on your ear training but also orchestration/composition etc. is to transcribe music by ear. Another important thing that helps to train your ear is to be very conscious about musical structures that you are playing or composing.
Memorize the impression a certain chord leaves, memorize how a certain tension note over a chord sounds , how a scale feels like etc. Being really fluent in these things helps tremendously to speed up your composition process as it will be more “knowing” than “guessing”.