Not too difficult. You make a currency that is pegged to either a basket of commodities, a basket of currencies, both, our even just gold.

When the price of the basket goes above the original price (inflation) you reduce the mining reward (or in worse cases remove some of the coin from circulation in various ways). When the price of the basket falls (deflation) you increase the mining reward increasing supply.

You would need a trusted oracle, but that is no different than any other smart contract.