In order to write a plagiarism free article, one of the most important things to do is you have to properly cite all of the sources that you use. Citing sources is very important because even if you paraphrase a sentence from an author without proper citation, that is considered as plagiarism. Also, make sure that your original idea is present which means that the majority of your work is not from other people’s works. Because if the majority of your work comes from other people’s writings, it is still considered as plagiarism even if you cite properly.
Therefore, I think to write a 100% plagiarism free article really takes a lot of hard work because you have to conduct a really extensive research. After you write an article, I think you can try to check it online but be aware that most of them are not free. Some are free but it’s limited to 1000 words etc. And the free ones are usually not accurate. These are mostly based on my experience whenever I write essays or articles for my study. You don’t copy or paraphrase someone else writing without saying where you got it from. It can be informal so long as you let the reader where to go to find your source.