Have something on your mind? Pissed off, or pleased, by something an author wrote in the Guardian? Doesn’s fit as just a comment, or you want more people than that to read it?
Just email the Editors.
Unlike print media, you don’t have to collapse your thoughts into any arbitrary length (e.g., 250 words). However, you’ll be more persuasive if you write succinctly and clearly. So don’t be so long-winded that your readers tire before the end!
Here’s a little more information.