develop as much of the history as you can.
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Think things out fully, i.e., will my world have a suite of gods, who are they?
What's the setting?
Where is it located, i.e. what type of landscape?
What's the population like?
What systems will I use, i.e. Crafting(Which), hunting, sickness...etc?
Is there a big bad guy?
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once you get all of your ideas down on paper, start thinking about if you can do what you wrote. For example:
you decide that your people are suffering from an unknown force that ages the population very fast, humans die out in a few months but some people are immune(PCs). The players are hunting for a cure, but while that is going on your npcs are getting old and dying so you need a system that will turn kids into adults into old people and then kill them off and replace them with new kids every 30 or so days.
it's an interesting idea but would be a pain to script and maintain.
Once you decide on your systems you need to find out if.....
A> The systems are out there for you to use.
B> You have somenoe who can make them for you.
C> You better learn scripting.
All and all, making a PW isn't hard...making a good PW is and it takes a ton of work...you must be willing to put in the effort and I hope be willing to stick it out for the long haul.