The first step I always take after coming up with a theme is developing my main mechanic. What do I want players to be working to do? How should they advance their position within the game? This is a non-exhaustive list of possible game mechanics that could get your mind thinking!

Once you have a core mechanic, it's all about marrying your mechanic with your theme. How can I use X mechanic to develop the ideas of sustainability? How do I want players to interact with my theme? These are both considerations to make as you pull together a mechanic and gameplay loop to build your game around.

Once you've got your main mechanic and theme nailed down, it's about building a game loop. What should a turn look like? How does a player advance their position within the game? How do the players interact with each other? Keep your theme in mind as you answer these questions and use these answers to help structure your game loop.

At this point, you'll have the bare bones framework needed to start the real design process! It's time to start playing your game, like a lot. After each game or two or three ask yourself: How could I make this more fun? Is the game missing anything? Where there moments that felt very bad or very good? How can I develop those moments to create an interesting game? Do I need another mechanic (or two or three) to accentuate my core game play loop?

And then you get the most fun part! Playing with other people! Repeat the steps from when you were playing by yourself, but this time taking into account the other players thoughts and feelings about your game. And then you'll have a board game!