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.
In fanfic, Naruto might be made to use the lean fighting style in several ways. Maybe he could incorporate its quick, evasive maneuvers into his ninja combat. For example, the lean fighting style's emphasis on using the body's momentum to dodge attacks could be adapted by Naruto, who is already very agile as a ninja. He could use it to quickly shift his body weight and avoid enemy jutsu or physical strikes.