Here are a couple ideas for some ways you might design your dungeon:
1. Predetermine the types and total number of monsters in the dungeon and roll 1d6 every time the hero enters a new room or corridor. On a 6, at least 1 monster is encountered. Roll 1d6 again for the number of monsters encountered:
D6 result
1-2 1
3-4 2
5-6 3
2. For quests that include the objective of returning to the entrance, the GM can roll dice after the heroes complete their turns to determine whether any monsters have found them. The GM can roll a D6, D12, or D20 where monsters are encountered on the last number on the die. An additional roll of a D20 can be made to determine the distance of the new monster from the hero.