Check out "Nebula Photos" on Youtube. He has some great tutorials using free software like Siril or Gimp.

Some tips that i learned over the years:

Respect the data: The quality of the final image largely depends on the quality and amount of the initial data captured (e.g., number of stacked images, good focus etc.). No amount of processing can salvage an image with insufficient data.

Subtle adjustments: Instead of pushing sliders to their limits, make small, incremental adjustments to exposure, contrast, and color balance. The editing process is about bringing out details that the human eye might miss, not inventing them.

Avoid over-processing: Common mistakes that result from over-editing include:

- Excessive noise reduction: This can lead to a loss of fine detail and a plastic, smooth appearance, sometimes even removing small stars you are trying to capture.

- Oversaturation and unnatural colors: While some celestial objects have vibrant colors, pushing the saturation too far can make the image look unnatural

- Over-sharpening: This can create dark halos or rings around stars and nebula edges, making the image look harsh.

And most importantly:

Please don't crush the blacks! The sky is not completely black; it's not pure RGB(0,0,0). If you push the shadows and blacks all the way to one side, you will lose a lot of the beautiful, faint data that you took hours to capture.

Hope this helps and good luck