One of the easiest ways to tackle writing a mission for a newbie is probably to break it down into chunks.
First, write out a list of stuff that needs to happen for your mission to work. Then break down those items into smaller chunks. Then smaller. You literally want to break them down as small as they can go, to individual actions.
For example, if you want to have a NPC ship dock at a starbase and upload a virus, and have the Artemis follow what's going on, you might break it down like this:
* Create NPC ship
* Create starbase
* Create Artemis
* Tell NPC ship to fly to starbase
* Tell NPC ship to drop its shields
* Broadcast a message to Science telling it that the NPC ship is transferring a lot of data to the starbase
* Tell NPC ship to wait at the starbase for a while
* Broadcast a message to Science telling it that the data transfer is complete
* Tell NPC to raise its shields again
* Broadcast a message to Comms which says "Ahahaha! You fools! Now I have infected your starbase, and the universe will be mine!"
And right there, you have a simple plan of some plot which takes place on a mission. The reason you've broken it down into chunks like this is twofold. Firstly, you've analysed in detail what steps you've taken to make something happen. You might even come up with new ideas to make the mission more complicated or interesting.
But the second is the most important as a newbie - it allows you to tackle mission scripting bit by bit. Because of the way the mission scripting works, you can go through each of those steps, learn how to script them, and then move on to the next. By the time you're done doing each step individually, you have a full mission.
The other piece of advice is not to make things too complicated for yourself. If you write simple missions first, you'll quickly learn how things work. Then when you move on to more complicated ideas, you'll be better at it and won't get stuck on things as much.
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