First you have to decide what skills you want to teach. Will the game give practice in performing that math skill, or will it just reward for correct answers? Those are two different things.
Then you have to choose some kind of a game format. Is there a skill element to the game itself? Is it practicing the math skill, or is it a game skill? Will it support or distract from the teaching?
Or is it just some kind of graphic metaphor—for example, computing the number of bricks in a wall—or graphic reward, such as gaining tokens for so many correct answers? Is speed a factor?
And then you have to have the design and coding skill to create the program, do record-keeping (scorekeeping), save data, etc.
This is a huge amount of work that entire teams of curriculum developers, media designers, and software engineers and programmers perform over a period of months in production settings. So perhaps this is not a right understanding of your assignment. Maybe a little more detail would help us help you.
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OPINION: Games that confuse practice with fluff and lengthen study time without contributing content or process knowledge are a waste of students’ effort, in my opinion. They don’t make learning “fun”—they confuse the real pleasure of learning and mastery with empty entertainment.