If I believe I have a high-grade copy of Detective Comics #27, what steps should I take?

First, handle the comic with extreme care, using archival-quality bags and boards. Do *not* attempt to clean or restore it. Contact a reputable comic book appraiser to get a preliminary assessment. If the appraiser believes it could be high-grade, submit it to CGC for professional grading and authentication. Be prepared for a potentially lengthy and costly process.