The only key to polishing your content writing skills is practice. Practice will help you improve your skills and also build your portfolio that you can showcase to get projects in the future.
Freshers can work with news editors or mentors from PR or journalism fields to get feedback on their writing. Every project that you complete will help to improve your skills. A good way to build your portfolio and get feedback is to take up a content writing internship.
The internship will help you learn important aspects of content writing, elements of good research, and also about various styles of writing that make you more employable. You will also get access to invaluable mentorship and feedback on your work.