option (i) have a credible centralised monetary authority that promises to redeem it for its reserves of dollars or gold or whatever you want it to be stable against. Though arguably for the crypto-currency to actually be credible it needs that monetary authority to hold (or have the facility to borrow at short notice) the asset it's backing it with

option (ii) replicate the fiat system: i.e. the spendable currency is mostly credit with a future repayment obligation and thus actual future demand to hold the currency, and imbalances between rates of borrowing and repayment which drive the currency value up and down are kept in check by algorithmic adjustments to "interest rates" creditors face when borrowing to meet margin calls and "taxes" which make debtors have to buy even more coin to meet payment obligations. (This still isn't going to work unless credit creators are regulated though...)

whether it's still a cryptocurrency after all that is another question....