Question

How to compose song on instrument like flute etc...

Answer 1

From personal experience I learned that the best way to compose on a instrument that you're new to, is to listen to songs that involves it as the primary instrument like solo or something.

Asking a friend, who knows to play that instrument, to play some notes and songs in front of you is also helpful. Give yourself time to feel its essence. Every instrument is capable of producing an emotion. Take time to know that feeling. Listen as much as you can.

Once you've listened to many songs and you feel confident enough, play the track that you're working on. You'll surely get some basic tune in your mind on the instrument you want. There are some special tricks and stuffs involved in all instruments that you might not get on the first go. But as you keeping refining the basic composition that you made, you'll learn more. Idea will keep coming. Every once in a while, like when you're going to bed, listen to some great compositions that involve that instrument.

Listening is the best way of composing!