You need to make it attractive by searching interesting story/flight plan, and location.
You search for scenery or make your own.
And then you start writing... well sort of, after all it is in XML format.
Now here is the difficult part, QA your mission.
I know that flying your mission can consume lots of time and after many tests even tiresome. But remember, when you publish your mission you should be as sure of your self as possible, that you at least made the mission goals clear and if there is a tough step, then clues should be added to make it enjoyable, it can be visual cue, map or any other means that will be accessible to the simmer.
The reason I'm writing this, is that I had an opportunity to read an interesting mission, but some of the elements definition were not on par with the "Designer Manual". I did not test the mission, but the issues I found seem to render the mission unplayable, or at least it wont reflect what the "author" meant.
This is why I created few tools/workaround for the designer, embedded in the plugin.
The first tool: Debug Widget - Displays constant stats of the current "step".
The second tool: Gauge Widget - Displays some coordinate info.
The Third tool: Though not a tool but an Element in the mission file:
<designer force_step_name="[step name]" ...
This is one of the most important element for testing your mission. You can start from any step just by modifying this element with the correct value (There is an activation element, but you should read the Designer Manual first).
This way you do not need to fly the whole mission from start, Just remember to disable this element before publishing the mission.
The forth tool: Visual Cues. Since v2.0x was published, I added in game visual cues, using simple OpenGL directives. The designer can now "see" where triggers/targets/feedback and more are placed in the XPlane world, and when the simmer activates them. Think of it as a visual debug system, but again, you should turn this feature off, before publishing your mission.
Remember the simmer should enjoy the mission and should not be puzzled why things does not work as expected, it is up to us to make the mission enjoyable, so take your time before publishing and run some tests, or find someone who will be willing to test your work.