Cleaning a scan basically means making the scans look good.
There are many steps in the process of cleaning a scan:
1. Colour-settings: Make sure the scan is in greyscale mode or, for colour images, RGB mode. [+] [-]
2. Rotating: Rotate the scan to have it aligned correctly. [+] [-]
Here, pnyxtr's rotating tool will be presented because it is, especially for batch rotating, much nicer than the Photoshop version. Just read his readme: [link]
To check if your rotation is correct, draw a rectangle around or beneath the panels and see if the rectangle touches the panel's outlines all the time or, at least, on symetrical places. If so, the rotation was done correctly (On the example images i filled out the rectangles, you obviously do not need do so when cleaning).
If you are working on low quality raws, do not rotate the same image two times but reset to before the rotation and do it correctly this time. If you do not do so, this will cause your cleans to be overly smooth, which especcially destroys patterns.
(On the left you see a page rotated one time, on the right you can find the same page rotated four times).
3. Merging: Connecting two pages that are part of a double page spread. [+] [-]
4. Cropping: Crop out unneccessary parts of the scan. [+] [-]
5. Leveling and dusting: Level the image so the blacks are black and the whites are white without damaging the greys then clean any specks that were left. [+] [-]
6. Redrawing: Complete any necessary redrawing, e.g. restoring the background behind offtexts. [+] [-]
7. Resizing: Resize your image to have a web-compatible size (depending on your group this may not be wanted) [+] [-]
8. Cleaning bubbles: Clear the text from the bubbles. [+] [-] [Action]
9. Saving: Save the image. [+] [-]
Other useful stuff pt.1: Adjustment layers. [+] [-]
Other useful stuff pt.2: Training Raws. [Low Quality]