My two pence (UK):
First, get a really, really good idea of the feel of your plugin, and the underlying storyline.
Next, use EV-Edit to make any stuff like Spobs, Systems, Outfits and Ships that you know you'll need (or, just really want to make). Reuse graphics from other stuff to begin with -- you can make the graphics later when you've got more of an idea of how you want things to look.
Always back up a lot when using EV-Edit -- it crashes like crazy and takes your plugin with it.
Then, convert your plugin to EV (rather than EVO) and create all the missions. While you're doing this, you'll probably want to go back to EV-Edit (which can work with EV or EVO) and add outfits, govts, spobs as you go along. Remember to back up like crazy. Still don't make any graphics -- EV-Edit and Schmelta are much more likely to crash if the file is large -- and that's exactly what graphics make it.
When you've written everything you can think of, download PlugChecker. PlugChecker will find loads of errors -- most of them created by EV-Edit. You can only fix these errors using ResEdit, but by this time it shouldn't seem too weird (something tells me that EV-Edit is heavily based on ResEdit).
Fix the errors before you do anything else (including playing your plugin).
Then, delete EV-Edit from your hard drive. At this point even opening your plugin in EV-Edit can cause endless problems.
From here on, use EV-Developers Map, plus some other excellent utilities made by Pontus Ilbring, plus Schmelta and ResEdit.
Play through your plugin five or six times. You'll expand it as you go.
When you've finished expanding it, run it through Plug Checker one more time.
Ok. You can now start making graphics in your favourite 3d rendering package, and sounds in your favourite sound editor.
When you're totally ready, advertise on this board for Beta Testers. Take no less than five and no more than fifteen for an average plug-in. Get them all to sign up to regular updates over (say) a three week period.
Everything they criticise, fix, no matter how much you don't want to be criticised. These guys are working for you, and you owe it to them.
Take especial care to fix any bad grammar, spelling mistakes or clumsy writing. Players will forgive anything but this.
Now work out how much space your plugin is taking. Ask yourself: is it really worth five hours of someone's time to download my 48 MB plugin if the total playing time for all the missions is only 25 minutes? If not, then mercilessly prune away graphics that aren't essential, and sounds and music tracks. You can always release them as add-ons later if your plugin takes off.
Once your plug-in is thoroughly ready, upload it on the add-ons page, and tell everyone on the board. Most people seem to immediately rate their own plugin five out of five. This is fair enough if you really think that your plugin is (say) better than Babylon 5 or Star Wars. A better way would be to ask people to give an honest rating after they've played it.
If the ratings are low, find out why people don't like it and fix it. Often the 1.0.x versions of a plugin are a lot better than the initial versions.
If there are bugs, deal with them immediately. No one likes buggy plugs.
If the ratings are high, remember to answer all the emails you get, and start work on the sequel.
Good luck.
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M A R T I N T U R N E R