Quality checking is the last and often most exhausting part of scanlation. A quality checker needs to find all mistakes made by anyone working on the chapter and fix them (or make somebody else do so).
You will obviously be required to be able to spot all of the mistakes and know how to fix them. This means most quality checkers are often a jack-of-all-trades; so if you want to qc and are not familiar with a step in the scanlation process yet, check out the other tutorials.
Here a suggestion on a workflow for quality checking will be presented:
1. Check if the page is supposed to be part of a double page spread (people really forget to merge pages sometimes...). [Cleaning - Merging]
2. Check if the page size, the cropping, and the centering are done correctly. [Cleaning - Cropping]
3. Check if the page is rotated properly. [Cleaning - Rotating]
4. Check for dust left over by the cleaner. [Cleaning - Leveling and dusting]
5. Check if there are any mistakes in the english. [Translating & Proofing - Using proper english]
6. Check the text's shaping [Typesetting - Centering, shaping and font-sizes]
7. Check the font-sizes, margins to the bubble's outlines and the shaping. [Typesetting - Centering, shaping and font-sizes]
8. Save the image as .png (greyscale pages) or .jpg (color pages). [+] [-]
9. Pack the whole thing into a .zip file. [+] [-]
Generally it is really important to have consistency in your releases. For example: don't have different fonts for the same purpose from chapter to chapter. You should check such consistencies (e.g. "has the right font been used") whenever they fit into your workflow.