Step one is recruiting some people to be a "customer" of your game. You give them $500 of dirty money, they go visit various retail establishments and buy $450 worth of pre-paid cards, then spend the balances on those pre-paid cards buying stuff from your game. Various banking institutions and app marketplace owners take their cuts, you wind up with clean money earned from selling premium coins to your "customers", which you then report as taxable income and use for whatever legal-market purposes you want.

So, the short of it is that it's a two-part scheme. Part 1 converts undocumented cash into pre-paid debit cards. Part 2 converts the balance of those cards into business income from selling digital goods. The receipt of cash gets offloaded to retail establishments that routinely handle such transactions. The delivery of goods is cheap and verifiable, and the payment for those goods goes through legitimate channels.

Compare to laundering money through a literal laundromat. If you double the sales through putting dirty money into the laundromat's cash deposits, you ought to be using twice as much power and water. Auditors can find that out. Selling twice as many virtual battleships, though? You've got customer accounts you've "delivered goods" to.