Write some client code for how you want to use the API and then elaborate those examples by making references to the spec. This might seem backwards to you but

Write some client code for how you want to use the API and then elaborate those examples by making references to the spec. This might seem backwards to you but you'll discover all sorts of edge cases and awkwardness in the process that you wouldn't have discovered by just writing the spec. You don't need any special tools to do that although apiary comes to mind as something worthwhile.