Reason to write:

• share information
• learn about something
• side-effects such as help to establish your credibility

How to write for developers?

Consider the clarity, personality and the uniformity of content. Each medium has it scale on these three.

The physical act of writing? Start with notes and expand. Just start writing. A similar approach is reported by Finding comfort in the chaos: How Cory Doctorow learned to write from literally anywhere.
The point is to have something, then iterate.