This is the question I get asked the most, but I don’t have any special secrets:
1) Before you scan, use slips of paper to frame your image, confirming that there’s a good rectangular composition that won’t have extraneous elements in it.
2) Press your comic book page absolutely flat against your flatbed scanner by piling some flat heavy objects on top of it (I use books). The smallest ripple in the comic will result in extreme blurriness. I often fold back my comic, so I’m only pressing a few pages to the scanner bed, and sometimes I put a pane of glass between the comic and the books, to ensure total flatness.
3) Scan at a high resolution - at least 1000 dpi. I go as high as 3000, if the area I’m scanning is really tiny. (Beyond that point, my scanner doesn’t capture anything extra.) You can always reduce the file size after you scan. The main thing is to avoid having your scanner make any decisions about what color it is seeing; you want to capture every shade of the overlapping 4CP colors and every granular detail of the paper.
a) Include some unprinted (“white”) area in your scan, even if you plan to crop it out later. This is important to preserving true comic book color, when applying filters with a program like Photoshop.
b) I keep all my scanner’s filters turned OFF (sharpness, color correction, etc.).
4) I then typically apply the Photoshop filter “auto levels.” This sets the whitest spots of paper to true white and the blackest areas of black ink to true black and modifies everything else based on those benchmarks. (Remember, there’s hardly any true white or true black on an old comic book page - there’s an infinity of grays and tans.) The result is an image that stays true to the comic book colors and that preserves the grain of the paper, but that is brightened up considerably.
5) Crop the image until you love it.
6) Post it on internet.