Each party signs every page of two copies and gets one signed copy. It could be a lot more signing, but you might consider four copies and each party gets two.
Contracts should be justified so the words go the end of the line. That prevents insertion of words. The left and right margins should be the same on every page.
The text should start at the same vertical position. The text should end at the same vertical location. The bottom of the text on any page where the text doesn't go to the regular bottom should be marked.
The pages should be numbered. X of Y is better than just X.
The pages should have some identifying mark or text in at least one of the corners, like right or left header or footer.
Paper security should be acid-free with watermark or better. All pages should be in the same directions, front and back, top and bottom. http://www.secureguardpapers.com/sg-originaldocument.html
Sign with “check-safe” pen that is permanent, waterproof, and acid free.
Scan the contract and email a copy of the signed contract to each party. (Forged email headers are easy for forensic investigators to detect.) https://articles.forensicfocus.com/2019/02/15/email-forensics-investigation-techniques/