Hi @Praneet_Thakur, I'll take "iPhone 7 Accessories" keyword for this.
This is a popular keyword, but since 2017 is more about the 'next' iPhone, we can make use of this 'keyword' to rank somewhat easily.
1) Write at least ten genuine reviews for the most popular iPhone 7 (or just iPhone) Accessories. Like, reviewing them individually first, one-by-one.
2) After that, make a 'list' post covering all those reviews, keep the title as "Best iPhone 7 Accessories You Should Buy" or something you like, but optimized with the required keywords.
3) What I mentioned 1) and 2) are the essential ones, but, after this, you need to interlink this 'list' post with all the other 'iPhone' related posts on your blog first. Take Wikipedia for inspiration, interlink as much as possible on your blog, do this naturally though.
4) Last but not least, link building. You can do this via blog commenting and guest articles on other relevant blogs, especially the ones writing about iPhone or Apple in general.
5) The 1st four steps are to rank "iPhone 7 Accessories", but the key trick to rank any keyword is, to make your blog better overall, regarding branding and authority. Take "WPBeginner" for example; it's probably the best blog about WordPress on the internet, even better than WordPress official Codex. So, make your blog as an authority in that particular niche, and you can easily rank your posts. Best wishes.