One way to write a good 'baby making twilight fanfiction' is to build on the existing themes in the Twilight series. The idea of love and sacrifice is prominent, and this can be applied to the concept of starting a family. You can also draw inspiration from the beautiful settings in the books, like the forests in Forks. When writing about the 'baby making' aspect, you can be creative with how the characters communicate their desires and fears. For example, Bella could write in her journal about her hopes for a family with Edward, and Edward could express his concerns through his music. Additionally, think about how the family would be accepted or not by the different vampire covens and the werewolf pack, which can add layers of conflict and intrigue to your fanfiction.