Explanation: Choose a model song, if you wish, that represents the sound you are after. The instrumentation you will decide upon determines the sound you will get. Feel free to add or subtract whatever you wish. Some songs sound better with just an acoustic guitar, for example. Others need a big band.
One thing to keep in mind, though, is your ability to perform the music live. If that is your aim, don't add too many parts to your recording that cannot be reproduced in a live concert. People might be disappointed.
Try to break out of standard modes when doing instrumentation in your music arranging. Not every song needs drums, and there are a lot of non-standard instruments that can add much. Nothing " has" to be there, and nothing is forbidden, unless it's completely unrealistic to recreate live if you need to. Ask yourself about each instrument if it is helping the song tell its story or not. Then decide.
Next: 3. Create a chord chart.