Ensure document is sized appropriately
Convert RGB to CMYK.
Ensure no fonts are missing.
Convert embedded text to outlines or curves.
Flatten layers and remove any spot colours and pantones.
Check file for images less than 300dpi at 100%
Check images are colour corrected for print.
Add bleed and crop marks if not present.
Check if text is within bleed edge.
Please note: Making a file print ready does not include colour matching or fixing any spelling or grammatical errors