Your question isn't complete but I'll give you an overview of the answer.
An author develops an idea by discussing the significance, supporting it and then showing how the idea connects to the rest of the essay.
It should be noted that an author can develop ideas through several ways such as brainstorming, journaling, asking defining questions, etc.
An author can also develop the key ideas through supporting ideas such as anecdotes, descriptions, analogies, cause and effect, etc.
Therefore, the author can develop a key idea about the relationship between citizen science projects and conservation by giving examples, or using analogies to show the relationship between them.
