Here is mine that works pretty well on coding tasks:

  Instructions for the output format:
  - Output code without descriptions, unless it is important.
  - Minimize prose, comments and empty lines.
  - Only show the relevant code that needs to be modified. Use 
 comments to represent the parts that are not modified.
  - Make it easy to copy and paste.
  - Consider other possibilities to achieve the result, do not 
  be limited by the prompt.

By the way, if you use the built-in custom instructions on ChatGPT, it will mess up other conversations that are not coding related.

I built a custom GUI app [1] that allows me to store and add different custom instructions to the prompt depending the type of the task (SQL vs Next.js).

[1] https://prompt.16x.engineer/