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. [+] [-]
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]
This is the process of making boxes and bubbles "blank". For the typesetter it is important to be able to look at the raw text easily, so you will need to create a new layer (Controll-Shift-N). Now you can just go over the bubble's text, using the brush tool (B), of course the brush color needs to be the same as the background color of the text, so usually your brush (and by that foreground-) color should be #ffffff meaning pure white.
A faster and better way to clean bubbles is using actions, for a bubble mask actions, check out this (just Google "Photoshop actions tutorials" on how to use actions).
Anyway, here we have the cleaned page:
9. Saving: Save the image. [+] [-]
Other useful stuff pt.1: Adjustment layers. [+] [-]
Other useful stuff pt.2: Training Raws. [Low Quality]