To edit your writing well, you need to understand some basic concepts about writing. Sometimes these concepts are called grammar, mechanics, or conventions. Sometimes they are called rules, but really these rules are just patterns that good writers try to follow so they clearly communicate what they want to say to readers.
Anybody can be a good editor. First, you have to know what's important and then you have to reread your work carefully. Everyone—especially teachers—appreciate a writer who rereads and corrects his or her work.
Handbook: The How-to Go-to Place for the Facts about Conventions
Have questions about the mechanics, usage, grammar, and spelling rules while you're editing? View these quick videos for help with a variety of writing rules.