When you score a critical hit, you get to roll extra dice for the attack’s damage against the target. Roll all of the attack’s damage dice twice and add them together. Then add any relevant modifiers as normal. To speed up play, you can roll all the damage dice at once.
For example, if you score a critical hit with a dagger, roll 2d4 for the damage, rather than 1d4, and then add your relevant ability modifier. If the attack involves other damage dice, such as from the rogue’s Sneak Attack feature, you roll those dice twice as well.
Rolling 1 or 20. Sometimes fate blesses or curses a combatant, causing the novice to hit and the veteran to miss. If the d20 roll for an attack is a 20, the attack hits regardless of any modifiers or the target’s AC. This is called a critical hit. If the d20 roll for an attack is a 1, the attack misses regardless of any modifiers or the target’s AC.
RollQuest ships the D&D 2014 SRD as a content pack, on a 5e character sheet that does the maths - roll from the sheet and let the numbers work themselves out. Free to start.