How to use old doors for privacy outside?

Unless the 'old' doors were originally exterior doors, they will very quickly disintegrate and rot if used outside.

Are you looking to create a fence with them? You could line them where you are looking for privacy and secure them into a bucket with concrete

Turn old closet doors in outdoor privacy screen:

https://tatertotsandjello.com/lowescreator-make-diy-outdoor-privacy-screen-subway-tile-planters/

Use old doors as privacy screen (Hometalk):

https://tatertotsandjello.com/lowescreator-make-diy-outdoor-privacy-screen-subway-tile-planters/

One of these photos has cool hinges, and they’re movable. Or like another you make a frame and hinge them to the frame, or onto posts.

I've seen whole fences made out of old doors you would just have to make sure to support them well with either T posts or 4x4 posts dug well into the ground. https://www.youtube.com/watch?v=0mV0pg-CEdk