Begin by researching existing cultivation system concepts in other works. This can give you inspiration for your own system. After that, outline the plot. It could start with the discovery of the protagonist's cultivation talent. Then, introduce the challenges they face right away, such as a lack of proper guidance or limited resources. As you write, keep in mind the balance between the cultivation aspect and the character development. Don't just focus on the power - up moments, but also how the character grows as a person during the cultivation process.
To start a cultivation system fanfic, you need to have a clear idea of the world - building. Consider the resources for cultivation, like special herbs or magic stones. For the characters, not only the protagonist but also the supporting ones. They can have different motives for cultivation. For example, some might be in it for power, others for knowledge. Also, plan out some initial conflicts, like a local bully in the village where the protagonist starts their journey.