I can't give you much insight on how the main study articles are prepared, but I can tell you how many of the auxiliary articles and feature articles in the Awake used to be prepared when I worked on a remote writing desk in my country.
First we would come up with an idea for an article that we thought would have good broad interest.
Next we would do some initial research and put together a one page outline of the topic we had in mind, showing the main points we proposed to highlight and the thrust of the material we planned to develop.
That outline would get sent to the US head office for approval by the writing desk there. If the outline was approved they would send the outline back to our branch with a go ahead to write the article along with an indication of how long they would like the article to be and for which magazine... For example they might say they would like the outline to be developed into an Awake cover series of 8 pages.
From there we would do more research and try to find studies or quotes or statistics etc that we could use to support the points we were trying to make and we would incorporate this into a written article. Then we would send that into the branch writing desk where another brother would edit it and send it back to us for a rewrite. Then we would fix all the issues and send it back for another round of editing before it was sent of to the US branch.
In the US brach the article would be further edited and possibly combined with other research that wsa perhaps done on the subject by another writing team in a different part of the world. Once the article was completed it would go into an index for possible use. A compiler would go through all the articles and select the ones that would go in any particular issue of the magazine according the the head office priorities or needs etc...
Hope that helps to understand the process!
Often we would come up with the ideas and other times the society would send us subjects to write about based on what Circuit Overseers suggested as needs in the field etc....