This is not my idea, I got it off TikTok.
Writing a letter to someone who's not in this reality?
Seems impossible, right?
Well... it's very possible!
All you have to do is write the letter in this reality, and then script they get it (add where they find the letter).
After writing the letter in your cr, destroy or lose the letter. So if you wrote it on your computer, delete the page. Or if you wrote it by hand, either put it somewhere you will probably never find it, or destroy it (throw it away, burn it, bury it, etc).
It's that simple, but...
Make sure you forget about the letter, if you are constantly thinking about what the letter said or if they will ever get it, they won't get it. Just believe they will get it but at the same time focus on other things.
Another thing you can do is in the letter write that when they get it to give you a sign to tell you that they did get it. One girl on TikTok told Fred to give her a message through red roses and then later that week she kept seeing red roses everywhere.
Resources used: TikTok