When I write my content I generally will put the KW in the title, first paragraph, and last paragraph. I also tend to use related terms in the body of the article, and determine this by how many words I am writing. I have to agree with many of the already suggested answers here. Be certain to always keep up with the changes google does, because what might be working today may not work in a few weeks.
However, saying that, some of the basics will always remain the same, do not over use the KeyWord's, do check your spelling, always try to supply content that will help to teach someone something new, that might be lacking in someone elses content you have read while doing your research for the topic.
When your building backlinks, take some of those KW'S and alternate them in the anchor text, so that it looks more natural, as to how someone searching for the topic your writing about would enter into their search box to get the result. Always put yourself in the visitors shoes when writing I have learned is the best practice, and SEO falls into line more easily.
I hope my answer helps your question