Few guidelines that I follow while coming up with a content are:
1. The content has to be unique, free from Copyscape.
2. It should be informative and not otherwise.
3. In mu opinion, keyword density should be lower than 1.5% of the no. of words.
4. Keywords should exist in the title of the content (if any) and it should also exist in the body of the content. Make sure the keyword is within the first 160 characters of yhe content.
I agree with original content, informative, with keywords.
HOWEVER--remember there are only 3 reasons someone writes: To inform, entertain, and persuade. You want to be all three! Your writing does not have to be as funny as Steve Martin or Bill Cosby, but you do need to keep the reader's attention long enough for them to get to the call to action (in other words, buy!)
Your content MUST BE correct--no spelling errors, no punctuation errors, no grammatical errors.
DO NOT TRUST SPELLCHECK! I can't emphasize this enough. Say, for example, you intend to write "from the halls of Montezuma to the shores of Tripoli" and you write "FORM the halls...". Guess what--spellcheck will not catch that.
Also, do NOT trust the grammar checker in your word processor. It can be useful, but it is not the be all, end all, word and the way of all grammar. There was a story, many years ago, about someone who ran Lincoln's Gettysburg Address through an early version of a grammar checker. For "Fourscore and Seven Years Ago..." the checker said something like "Too fancy. Use 87 years ago". It turned Lincoln's eloquence into boring pedestrian dreck. Trust your instincts.
There are some techniques you can use:
One technique to check your work is to read it backwards.
Another is to put your writing aside for a day, then reread it. This will not only help catch errors in speling, grammar, and punctuation, but will also allow you to catch those missing words you intended to put in, but did not.
A third technique is to give your writing to someone else to read. When they ask questions, note them and either reword it or answer the question in the text.
Finally, if English is your second language, and you are targeting native English speakers, see if you can find a native English speaker to read your writing and see how he/she likes it. Do not feel bad if they criticize your work; remember, that's what you asked them to do, and your goal is to sell, not increase your pride. Pride of authorship is NOT appropriate in this instance.
Always write a unique content of your own. Never copy from anywhere.
About keywords, The density should be well balanced. You can have as many times as 3%. But it should be well placed in the content.
Also about links, all links should be placed so that it looks natural outbound links.
Before writing always try to collect all details you get about the topic. Never write outdated information you have. So first you should update yourself about the latest in the topic.
First get your own overview about the topic. And then after if you have enough knowledge then ok but if you have not then research more about that topic. Do not copy from anywhere but write your content after research and write in such matter that people can understand your concept.