How to use ChatGPT API to perform very specific task?

I am new to using the OpenAI API and I want to use it to perform a very specific task inside my application, for example, to convert an input text with an instruction to a csv-formatted text.

How can I use the API to get the model to perform this task without passing it the instructions each time? As sending the instructions each time will clearly increase my token usage, isn’t there a more efficient way to do this?

Should i use fine-tuning to achieve this?

Thanks in advance.