Create a file named helloworld.pyx:

def example():
    print("Hello World")

Create a file called setup.py:

from setuptools import setup 
from Cython.Build import cythonize

setup(ext_modules = cythonize("helloworld.pyx"))

To compile and build the Cython file:

setup.py build_ext --inplace

You can now use your compiled Python extension via:

import helloworld

helloworld.example()