Going To The Underworld...

I guess I should mention the afterlife really quick, and then run. Before you throw your characters into the afterlife, either temporarily or permanently, you need to make some very basic decisions. These decisions are: If it is punishment/reward/both/neither, how long characters will remain there, what the rules are, and what its purpose is in your story. Let's deal with the last first, and then proceed in the normal fashion.

A sojourn into the afterlife needs to be a momentous event. It should not be chosen just as an interesting background, but because there is an actual reason for it. If the characters are living, the obvious reason is because they need advice from someone long dead and standard summoning just doesn't work, or there is an item or material they need for a special plot-needed item; they need to have a really good reason to be there. It can also be a reward of sorts, such as seeing a long-dead hero or relative in order to work things out. If the characters are dead and they are exploring the afterlife, there needs to be a reason, such as exploring the idea of final chances or to take a look at their lives, that they have yet to pass on to their final reward.

Once you have determined that, you need to decide if the afterlife is a punishment or reward, both or neither. It can be just way station on the way to another location, or way to deal with things before the person can rest, go to the next stop, be reincarnated, whatever. There just needs to be a reason for it to exist within the cosmological framework, and it needs to work with the purpose you need it for. Keep in mind that if the characters are living, it will serve as a preview of their own fates, and what they are ultimately fighting for, so keep that in mind as well. It needs to be more than just another place or you are just wasting it; it is sort of like using a tactical nuke to take out a fly, and that's just stupid.

It also helps if you decide how long the dead stay there. Sometimes it can be the final resting place after a single life or when enough reincarnations have happened, or it can be very short, like a depot as the souls wait to go to their next stop or must do something in order to leave. You can give them an actual time limit, a limited time, or somewhere in between. That time limit, if you have one, must be enforced somehow, so that's another consideration.

Once you have decided all of that, you just need to define what the rules are. It can be its own world, a parallel world of sorts, or even the regular world, but the rules of how the characters may interact with it needs to be defined. There is even the option of having the spirits interact with the real world but having their own world, making you need two sets of rules. The big one is whether or not the characters can be killed or not, and if so whether or not it is a permanent death or just an inconvenience.

Once you have thought all of these things through, you can start thinking about writing, but at least you have a better idea of what you are doing.