Do you have their urn or gravestone? That's the most important part, because first you'll need to get their ghost to appear. You can then make friends with the ghost, and get them to join the household-- even if they were in the same household before, they aren't anymore.
Once their ghost lives with you:
Do you have Realm of Magic? If so, it's so easy it's like cheating, higher level spellcasters can simply cast Dedeathify. Poof, they're alive again. You could visit with a spellcaster from another household, if you have RoM but no spellcaster in the household.
If you don't have RoM, it'll be a lot more work. You'll need to make Ambrosia:
https://www.carls-sims-4-guide.com/skills/gourmetcooking/ambrosia.php
