Once you have carefully selected your key words and phrases for your website, you now have to tune the website to match them. You need to carefully place these words and phrases into your website content, particularly in the non-visible areas, for example the various tags which help define what your site is about. Search engines take note of all these tags to help them decide where your site will be indexed. Links are particularly useful here, and people often get them badly wrong. If you use, for example, for more information about search engine optimisation click here, you risk being indexed for the 'click here' phrase and you will be competing with millions of others! The format you should use is 'for more information about search engine optimisation, click here.
There are also a number of other tags which should be used correctly to ensure that the search engines correctly index your website.
Experts differ on the quantity, but I believe that you should have at least five hundred words on a page for Google to recognise it as a valuable page and index it correctly. You also have to place the keywords and phrases in as natural way as possible within the document to make it relevant, the search engines frown on what they call keyword stuffing. If the opening sentence of your website is “widgets, widgets, widgets”, they will know that you are trying to fool them, you should sprinkle your keywords around the document in as natural a way as possible. Your should also use bold text and title tags in your document to emphasise the key words and phrases. It is also important to match the tags on your web page with the content, for example if you use 'widgets' in your tags, you should also use 'widgets' in your content.
Writing website content from the point of view of search engine optimisation is one of the most difficult aspects of search engine optimisation – many customers do not want their website home page cluttered up with too many words, but it is vital to get your website highly ranked in search engines.
Search engines do not like duplicate content – if you 'borrow' content from other sites, or use the same copy on multiple pages of your site, then Google will spot it. Google says that content must be 30% different to be valuable, how they measure it is anybodies guess, but never underestimate the power of Google! At the very least your content will be ignored, in the worst case your site will be downgraded because of duplicate content.
If in doubt, consult an SEO Expert!