I'm hoping this is especially useful for people who are stuck with a short sketch, and don't know how to make it sound better or how to make it longer.
(I might make this a regular series, where I take other people's compositions and suggest ways they can be improved and developed. Message me if you're interested in this!)
When you're writing, there are many paths you can take to develop the music, and the path you DO take, often forks and keeps on forking. So for the sake of this tutorial, I always start at the beginning, and I don't go too deep developing in a particular direction. I'm also not considering the music's overall function and context, like if it's part of a game. This way, the topic stays focused on the composition tips.
Tip #1 - Controlling your parts (i.e. bass part, drum part, melody etc). If you identify, separate and control your parts, you can introduce them at your will, and pace how your tune develops.
Tip #2 - Play with textures - often you can make what you've already written sound much better by adding things like volume tails, echos and playing with different effects.
Tip #3 - Add parts, not as a way to make the composition longer, but to make what you already have sound better. Can you add a drum/rhythmic part to contrast what you've already written? Can you fill melodic gaps with accompanying counter melodies?
Tip #4 - Expand the form of your tune using changes in melody, harmony and phrasing. I demonstrate this in two ways - with music theory, and without music theory. If you're not familiar with music theory, you can easily create contrast between sections by taking what you've written, and transposing it into different keys.