It's simple, you do it just as for any other freely traded currency pair, like EUR-USD - assign some body (central bank or some dept in ministry of finance) a duty to publish 'official accounting rates' and make a procedure on how to calculate such rates such as average rates of multiple large trading bodies.

It doesn't mean an offer to buy/sell at those exact rates (e.g., EUR/USD spot rates at every second will definitely be different from the 'official' accounting rates), but it's a guideline to evaluate value of things nominated in that currency.