To add a signature to forum posts, users should click on "My PF" in the upper left corner and select "Edit signature." However, only contributors and staff members have the ability to set a signature. To become a contributor, users can click on the "Upgrade" link next to "My PF" for more information. The discussion highlights the accessibility of signatures for contributors while providing guidance on how to set one up. Overall, understanding the contributor status is essential for users wishing to customize their posts with a signature.