Proofreading is the final step of the editing process and is done after the manuscript's final layout has been created. This step is to double check the work of the copy editor and interior designer, so if your manuscript hasn't been seen by both then you are not ready for proofreading.
Proofreading includes, but is not limited to:
Flagging typographical and formatting errors and irregularities
Checking consistency in layout
Checking for visual elements that will distract the readers
Checking for major errors in spelling, grammar and punctuation
Ensuring that all edits have been transferred to the final draft