Would you bother preparing something to help prove your reincarnation, or would you simply take advantage of whatever you retained (and prepared for yourself) from your past life?
How might you use this ability to help yourself and others?
Edit:
To clarify, suppose you have some vague memory of your past life in your reincarnated self that helps you along and allows you to take advantage of whatever your past self prepared for you.
Longshot: at that point I'd make a video basically doxxing my past self and adding as much useful information (who I am, where I lived, my lawyers name etc…) then upload that video to whatever video hosting service I think can survive long enough for reincarnated me to find it (hopefully they remember what keywords to look for or maybe the channel). For fun past me towards their eol would also start putting as much money into gold, bury that gold somewhere and leave a letter with a trusted third party, where they will then ask questions to whomever approaches for the letter to verify that I am the rightful reincarnated person. That letter, as you may have thought, would have instructions, a map and anything else needed to get past me's treasure. That should jumpstart reincarnated me's dream, whatever that will be