The most effective way to write for your website is to keep subject matter of your web page under tight focus. Notice I didn't say website; that's because search engines don't spider websites, they spider web pages.The search engines will look to see how well related page title, keywords and body text apply to subject matter. This means that if you are talking about widgets, your title, keywords and body copy should be tightly focused only on widgets. Stray from your main subject only when secondary ideas have very high relevancy and keep it short, sweet and get right back to your main subject matter.
To begin with, pick out your web page title and then a set of keywords that relate to your subject. After this, start to write body copy in a style you're comfortable with and do your first draft.
While working on your first draft, use this guideline:
Use a rough outline to get your point across. Have a start, a middle and an ending. Make your points in very middle of body copy, as I'm doing right here.
Don't limit yourself to a mechanical formula for creating a keyword density because it will get in way of your natural style.
Do your best to write in a free flowing manner as if you are talking to a good friend. Stay away from words or expressions that would make anybody reach for dictionary.
If you can get your style to loosen-up, have some fun as you write for readers and visitors. Do your best to have real you come through in your chosen writing style. This is something that will come on its own more you do it.
For second draft, do this:
Go over your writing and eliminate any fluff and revise it to make your meaning come through better. It could be there is an idea that needs expanding or perhaps you need to restructure a paragraph. Maybe there is another word that could provide a better description of an idea or term.