I don't know whether you aware of the extremly helpful articles in the AUM Magazines. Look at the small game samples to get an idea of the effort that you will have to make. Try to make a concept of your game idea that is as small as such games. Write your game concept in few sentences. Then write a sentence for each function, that you need to write to achieve the minimized version of the gameplay that you imagined. Then code and test the functions step by step until you've got the gameplay.
You might want to let others test the game - listen to their feedback: what did they understand, what did they misunderstand, what did they like etc.
Then start again with widening the concept to several levels or settings or whatever your 'bigger' plans are.
Don't forget to write down that 'extended' version of the concept before you go on with coding, modelling etc., as well.
You can call this repeating and extending prototyping.
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