Roll to hit. For attackers with 1 HP, roll one die (1d20). For attackers with 2 HP or greater, roll two dice (2d20). For each individual die roll equal to or greater than the to-hit value, a hit is scored.
So for your examples: 1. the die with a 1 flubbed, check to see if the other die hit 2. the die with the 20 is a crit, check to see if the other die hit 3. the die with the 20 is a crit, the die with a 1 is a flub
Also, if you roll two 20s only one counts as a crit.