I've come to Computer RPG (CRPG) creation from a background of writing and as a DM/GM of pen and paper RPGs. As the writer of a CRPG you are the GM of your game, however as you can't be there in person to oversee everyone who plays it you are providing your services through the ai and routines built into the game.
Before you go anywhere near an RPG creation package (RPGC) it will help you and make your game flow more smoothly if you have it all pretty much written before hand. Then all you need to do is transfer it over to whatever RPGC you prefer to use, or are forced to by your OS says the disgruntled Mac user. In fact using standard software prototyping methods you can, if you have friends who play CRPGs, test your game before loading up your RPGC, in fact you don't even need to switch on your computer.
The process of writing your CRPG needs to start at the beginning, and the beginning is story writing. There is a very good tutorial already on the importance of story writing, it can be found at RPGRPGRevolution, so I won't repeat it all, suffice to say that a CRPG can be likened to a 3hr epic, the story and plot are much more important than they are in a FPS or action movie. Whilst you can have a brilliant story and an awful game, you can never have an awful story and a brilliant game; your game will never be better than the story it tells.
Next you have to create the components of your game; you'll have done some work on this when writing your story. These are the building blocks which go together to make your game: Characters, NPCs, Maps, Monsters, Props, et al. you will find that one thing that will help a great deal is graph paper. Graph paper with a large grid is where you can plan out and design your maps, maths exercise books are good for this, whilst graph paper with a smaller, say 1mm, grid and some felt tip pens or coloured pencils can be very useful for designing graphics, sprites and map tiles, before going into your image editor and creating them.
During this stage in the process you will also need to design the monsters and antagonists that your player characters (PCs), will be going up against, you'll need to work out how strong they need to be and how many XP and GP they reward. You'll also design your PCs, how many XP they need to level up, how much their stats will go up when leveling up, etc. This will need to be balanced with the size of your game so that the PCs don't level up too quickly but also so that the player doesn't have to spend too long leveling up, as that will slow down the game and make it boring. when you've got everything roughly where you think it should be we move on to prototyping.
There are essentially two ways this can be done: using the computer, using pen and paper.
Pen and Paper MethodPerhaps, and probably surprisingly, the easiest way to do this is with paper cutouts, which can be hand drawn or printed out from the computer.
These will represent the interface and the various items, i.e, PCs, NPCs, Monsters, Props, Maps, etc. that will appear on the screen. However to do it this way you will need to have someone available to 'play' through the game with you. Using this method you will take the role of the CRPG and control things such as random encounters, combat, and so on, much like with a traditional RPG. This will allow you to make notes about things as and when they occur. However this can take a fair amount of time and effort. Also note that you would not want to try and play out too much of the game like this, maybe just some exploration and random encounters, including leveling up a couple or so levels, some interaction with NPCs and maybe part of a 'dungeon' and a boss battle, and maybe at a variety of levels. This will allow you to see how the various parts of your game work as they are played and to make any changes that need to be made on-the-fly.
The fact that you are involving a third party to play is important as they will not know how the story goes and where things are located, whilst you, as the designer, will have a biased approach to the game. Therefore, no matter which prototyping method you use you should always get someone else to play through the game.
Two Computer Methods
The other way you can prototype is using the computer. This can be done in two ways, one merely shows a player how the game will work and is exceedingly limited in what it will allow your victim, I mean volunteer, to do whilst the other will give a more accurate view of the game.
The first method is to use a piece of software such as a presentation package, like Powerpoint or Keynote, with slides sowing the interface, or something like it, which progresses through a selection of fixed paths through part of the game. Whilst this is fine for some games and software it will not give a wholly accurate portrayal of how the game plays.
The other is the traditional Beta/Demo version of the game, where you actually create a small version of the game, possibly jumping from one section to another in order to demonstrate different stages. This has the advantage of allowing you to get to work in your RPGC, which is always fun, allowing people to test it without you being there, and also allowing you to get started on the game its self, saving you time later. However it's main disadvantage is that you have to wait for feedback from your play testers before you can implement any changes that need to be made. Furthermore the fact that you've now started working in your RPGC can be distracting and you may end up spending time tweaking things and working on minutiae rather than working on getting the essentials finished.
It's a Development Cycle
Once the prototyping stage is complete it's time to go back to the beginning. This means if, going on the feed back you received, you need to alter any parts of the story then go and do so, then move on to the components, and then the prototyping stage again. You may have to go through this development cycle several times. However once it is all done you should be able to go straight into building your game knowing how everything should be. It should mean that your games become more polished and that less time is spent tweaking stats and maps to get things to work properly.
Thank you and goodnight
As I said before, I'm coming at this as someone who used to design scenarios for pen and paper RPGs and also as a writer. Whilst I find that this simplifies and streamlines the design stage of game development for myself I accept that it may not work as well for other people. However if this helps just one other person with their games then it has done its job.