Add Palpatine’s “Power! Unlimited Power!” line from ROTS to the end of ROTJ when he’s zapping Luke.
Add it to every single time he uses lightning in the saga. Vs Windu, vs Yoda, vs Luke, vs the Resistance fleet, vs Rey. Oh, and when his hands crackle in the opening of TROS, he whispers it in hushed tones.
Then do the same for Rey when she accidentally uses lightning in TROS, by chopping up Daisy Ridley voice clips from various sources. Rey will be confused why she instinctively said that phrase in that way, but keen-minded audience members who are familiar with the previous movies will recognize it as a subtle clue to the later reveal.
Whenever he gets stopped from using it, eg Yoda catching the lightning, or his own death, have him yell “Limited power!” by clipping the line and dubbing it in.