NOW here is a very useful website that will help you improve your written French.
The Le Patron website allows people to type in body of text and then scans it to check for errors.
For example, if you write …une petit maison, Le Patron will highlight the phrase and say the adjective in front of maison should be feminine petite.
The site will also tell you what the actual error is and explain how to correct your text.
The only problem I found was that it is clear the site is very popular and can fail to return answers so you may have to re-submit your text again.