Then start by writing a spec.md file where you describe what should be built. Write like you would write to a smart developer.

Then use the highest thinking model available with prompt "Create ExecPlan for the task @spec.md and write it to file". It will think a while and create the file.

Take a quick look at the generated file. It may have some open questions or surprises you want to review and write some answers to.

For the implemenatation I usually switch to medium. Then request with something like "Implement @execplan.md". If it has numbered steps, it seems to help to say "Impelement steps 1,2,3,4,5 and 6 on @execplan.md" - this way the agent is more likely to complete the whole plan in one pass.

[1] https://cookbook.openai.com/articles/codex_exec_plans