Critical Hit

When you score a critical hit, roll for damage normally. Then determine the maximum damage you can roll with your attack before applying any modifiers. Combine the maximum result with your rolled damage and then add any modifiers. This is your critical damage.
For example. A level 1 Fighter with 16 Strength, giving them a +3 modifier, wielding a longsword gets a critical hit. The damage die of the longsword is 1d8. The maximum damage you can roll is therefore 8. The player then rolls a 3 on the d8. The critical hit damage now is 8 + 3 + 3 = 14.
Another example is a level 3 rogue with 18 Dexterity, giving them a +4 modifier, wielding a light crossbow. On a critical hit they can use the maximum of both their sneak attack die, which is a 6, roll their sneak attack die and add onto that the maximum of the crossbow die and roll for it. The critical hit damage now is 6 + 6 + 1d6 + 1d6 + 8 + 1d6 + Dex (4) = at a minumum 27 damage.