Unless your sim is max skilled in both cooking and gourmet cooking then you cannot make ambrosia even if you did have the ingredients. Writing the Book of Life means you have to have completed the writing aspiration first, and grafting to get a death flower requires minimum level 7 gardening skill.
Then on top of that you cannot get your ghost to eat the ambrosia unless they are part of your household, and its unbelievably difficult to get them to rejoin the household!
And all this before 3.30am Thursday am when you dead sim gets culled anyway......... although there is a mod that fixes that
