At which stage of writing an essay would brainstorming be most appropriate?

The brainstorming comes as the initial form of writing, also called pre-writing. The four stages for creating a new work, especially of writing, are:

1. Brainstorming

2. Culling and selecting

3. Writing/implementing

4. Editing/improving

Some authorities emphasize that one of the secrets to finishing polished creative works is to keep steps 1 and 2 very separate, and again keep steps 3 and 4 separate.

In other words, don't cull (eliminate) ideas while brainstorming, and don't edit while you write. This keeps your energy focused and improves productivity and confidence, and avoids "writer's block".