I don't have enough context to determine exactly who 'garp' is. It could be a newly created character by the fanfic writer, or it might be a character from a relatively unknown story that is being incorporated into the 'ace fanfic'.
First, you need to create a compelling original character (OC). Think about their personality, backstory, and what makes them unique. Then, plan out the training regimen that the GARP will use. It could include things like martial arts training, survival skills, or leadership development. For example, if your OC is shy, the GARP could use group activities to help them build confidence during training.
Typically, a story in a building is around 10 - 14 feet (3 - 4.3 meters) in height. This can vary depending on the type of building, whether it's residential or commercial, and architectural design.
I'm not entirely sure specifically what 'garp save ace fanfiction' is about without more context. It could potentially be a fan - made story in which Garp tries to save Ace within the context of a particular fictional universe, perhaps related to One Piece.
Character development is key. Fanfictions explore how Ace and Luffy grow under Garp's influence. Maybe Luffy becomes more disciplined in some stories, or Ace learns to be more open - hearted, all thanks to their interactions with Garp and each other.