Lock an adult sim in an empty room and use cosmic curse on them.

Wait a few days until they starve and the Grim Reaper will show up.

They have 5 days to live as a ghost before they fully die, so fulfil their needs & get their job back ASAP (they automatically lose their job when they die).

Find an adult sim of the opposite gender who has a good relationship with your ghost. Try for baby with them. When the baby is born, it will be a ghost!

If you have 3000 simoleons, get the moodlet manager and cure your ghost every time you play and they can live as a ghost for as long as you want.