CBZ is a standard file format used for distributing comics. The file format itself is quite simple, it's just a .ZIP file full of sequentially-numbered images (usually .JPG).
Here are some tips for how to take scanned comic art and clean it up for inclusion in a CBZ collection, using Photoshop.
Step 1: Scan
Comics are usually printed using a "Halftone" process. The image consists of different sizes of dots of the four process colors (Cyan, Magenta, Yellow, and Black). From a distance, they look like different hues and shades. (This is different than pixels, which are all the same size and have different brightnesses of the color components).
When scanning, I like to scan at a high enough resolution that I can see individual halftone dots, so I know I'm not losing too much detail. For the project I'm working on, that means scanning at 600dpi. Later, we'll resize the image so that those dots join together into areas of smooth color, but we want the starting artwork to be sharp.
It's sometimes tricky to get the pages to lie flat without taking apart the source. I've been using a flat clipboard and a scanner that I can place on top of the original.
Step 2: Clean Up the Color With Levels
If you're scanning an old comic, it's likely that the page has discolored a bit with age. You can fix this and some other problems with the Levels command in Photoshop.
Use the Image > Adjustments > Levels... command. Select the white eyedropper and choose a color to treat as white. In comics, the gutters between panels are usually ideal.
While you're in levels, you can adjust the overall brightness/darkness of the image. This old page didn't have very solid blacks, so I've moved the black point to the right.
Step 3: Clean Up the Gutters With the Magic Wand
You may need to further clean up the page background. Use the magic wand tool to select the gutters and delete them (fill them with white).
Step 4: Use Perspective Crop to Straighten
If your scanner wasn't perfectly aligned or the art was printed skewed on the page, you may need to straighten the art. Use the Perspective Crop tool.
Choose the Crop tool and make sure that the Perspective option is on. This will allow you to drag each corner anchor around independently.
Drag each corner anchor to a corner of the art on the page.
You probably don't actually want to crop to this area, because things may be sticking out (in my example, some word balloons protrude outside of the frames). Grab the center anchors and drag them out to the edges of the document. The skewed angles will stay the same as the center of the crop area moves.
Step 5: Resize Art and Copy Into a Fresh Page
Now that we're done editing the art, we can drop the resolution to our output resolution. Use the Image Size command to change the resolution.
We'd like all of the pages in our CBZ to be the same size, so make a new empty page the size of the original page (in my case 8.5" x 11"), and copy the art into this page.
Save the file and start working on the next page!