Scope: Your topic should be broad enough to appeal to a variety of interests but narrow enough to allow you to fully cover it.
Avoid Preemption: Your topic should not repeat previous scholarship. It should contribute something new and non-obvious to the conversation.
Allow your topic to evolve as you research: The initial formulation of your thesis and supporting arguments should develop as you learn more about your topic. It may be that you encounter better arguments or find objections to your initial framing. You should allow your thesis to change if the evidence dictates.
Often academic papers begin with a literature review to orient the reader to the scholarly conversation thus far. The preemption check, done while choosing a topic, can be the start of your literature review.
In addition to researching the academic literature around your topic, you must research the law. If you topic involves a statute, find the history of that statute's passage and its enforcement, contextualize it by looking at other jurisdictions and identify sample cases on point.
State your thesis and then enumerate your arguments for that thesis.
Each argument must be supported by evidence.
Anticipate objections and explain counter-examples
Once you have articulated your arguments, you may need to research again in order to find the best supporting evidence. Choose the most authoritative evidence available.
The first draft can be very rough, the important thing is to get out your ideas. After that edit for clarity, organization and concision.
Where possible get feedback from colleagues or mentors. Be appreciative of feedback and consider it carefully. Create and number versions of your paper to refer back to when necessary. Keeping distinct drafts will also help you to understand your writing and see how your ideas have evolved.
Before finally submitting a piece of academic legal writing, research again. Make sure nothing noteworthy has occurred during your writing process and if there have been new developments, acknowledge them.