mood – to recreate mood first submerge yourself in it. Then use your knowledge of the medium you are comfortable with to write it.
If you want to write a sad song then go and get hurt and remember all the sad stuff that has happened in your life. Listen to all the sad songs that you like. Watch all the sad paintings. Read all the sad stories. Do everything and anything to reach that deep sadness residing inside of you. Then let it grow. Let it become big like a hot air balloon until it burst open your rib cage. But hold it back with all your might. Do not let it go. Hold it as if it’s the arm of your lover falling off a cliff. Hold it and place it in the deepest corner of your body and let it burn your insides. Then one day you will lose. It will disobey you. No matter what you do it will defeat you and that day you will win. On that day you go grab a pen or a pencil or a rock and write it down. Record it before you come to your senses.