Option 1: At the start of each round, one dice is rolled for the players and one dice is rolled for the referee’s creatures. Ties can be re-rolled or may just indicate that each action occurs simultaneously. Whichever side gets the higher score goes first. The players can resolve their actions in whatever order seems appropriate (perhaps starting with the character who has the highest dexterity, then the second highest, etc.). The referee resolves the monster attacks in whatever order seems appropriate. After everyone has had a chance to perform an action, the round ends and a new round is begun.
Option 2: 1 dice is rolled for each individual character and/or each monster type (thus the DM might roll one dice for all of the orcs and another dice for all of the ogres if orcs and ogres were attacking together). One starts at the highest score and counts down to the lowest, resolving all actions in order. The type of dice used is based on the creatures dexterity; average monsters and creatures might use a d6, faster ones use a d8 or d12 and slower ones use a d4.