Choose the instrument first. XAUUSD spot is the retail default at 0.01 lots per ounce; COMEX GC futures are 100 ounces per contract and micro gold (MGC) is 10 ounces.
Then read the broker gold specification, size the position from the stop distance (lots = risk divided by stop in dollars multiplied by 100 ounces), choose a single session, stay flat into US CPI, FOMC and Non-Farm Payrolls, and journal every gold trade with session and volatility tags.