Consistent Critical hit damage

When you score a Critical Hit, you double the damage dice as normal. You then select 1 die from each source of damage and treat the die as if it rolled its maximum value. You then roll the remaining dice and add any applicable damage modifiers.

Small Example:

A Fighter with an Attack of 1d8+5 scores a Critical Hit. To determine the Consistent Critical damage, you double the dice as usual to 2d8, but you MAX one of the d8’s. This means one of the dice is just a flat 8 and then you roll the remaining 1d8 and add your +5 modifier. The total damage is 1d8+8+5.

Big Example:

A monster with a Claw Attack of 4d6+5 scores a Critical Hit. The creatures 4d6 are doubled to 8d6 as usual, but you MAX one of those d6. This means that you only roll 7 of the 8d6 and the 8th one is maxed out to be a 6 (meaning you don't need to roll it), then you of course add its +5 Modifier. The total damage would be 7d6+6+5.

Complex Example:

A Paladin Rogue multiclass gets a Critical Hit with their 1d4 dagger, 3d6 Sneak Attack, 1d8 Smite damage, and +5 Dexterity Modifier. The dice would double causing the damage to be 2d4 + 6d6 + 2d8 + 5. You max out one of EACH source of damage: the dagger, the sneak attack, and the divine smite. You don’t roll this maxed out dice, so the final damage result would be: (1d4 + 4) + (5d6 + 6) + (1d8 + 8) + 5. 

You max out one dice from each source of damage. If you had poison on your blade you would max one weapon damage die and one Poison damage die. If a Dragon’s Bite Attack had Piercing damage and Fire damage, one die from the Piercing damage would be maxed and one die from the Fire damage would be maxed.

Tip: 

Gather all your dice in your hand. Then to MAX a die just, take one out of your hand and set in on the max value. This is a very clear way to see it all happen, then calculate the damage and add it up as usual.