It seems really easy to use: Copy or paste the text you want to check in the text box and then click the button .
If there is a problem with the sentence you wrote, http://www.grammarcheck.net/ will underline in green the incorrect part. You just have to click on the underlined phrase for GrammarCheck.net to tell you what the problem is.
Remember the other grammar sites that this blog features.
Grammar Speaks (Azar Grammar)
H/T to Technology Tidbits