Begin by doing some research on the 'Mew Mew Power' universe. Know the rules, the existing characters, and the overall tone. Next, create a detailed backstory for your OC. This can include how they got their powers and what their life was like before. Once you have these elements, start writing the first chapter. It could be an introduction to your OC in their normal life before the Mew Mew power stuff starts to happen. You can gradually introduce the elements of the 'Mew Mew Power' world as the story progresses.
First, think about your OC. Decide on their appearance, personality, and powers. For example, if your OC is shy but has super strength, it can create interesting situations. Then, plan the plot. Maybe start with your OC discovering their powers. Keep it simple at first.
To start writing a 'Mew Mew Power OC Fanfiction', you should start with a concept for your OC. Consider their role in the story, whether they are a hero, a sidekick, or something else. Then, think about the conflict. It could be an internal conflict within the OC, like struggling to control their powers, or an external conflict such as fighting against a new enemy. After that, you can start writing the opening scene. It might be a good idea to start with a scene that grabs the reader's attention, like a mysterious event that leads the OC to discover their Mew Mew powers.