1. If it's a stat that you're testing against (say, you need to make a PP check), you roll d20 and have to get under your stat to succeed. A natural 1 is always a success, and a natural 20 is always a failure.
2. If it's an opposed roll:
- If the players have the same stat, they roll d20s.
- If one player has a higher stat than another, both players roll a d20. The player with the lower stat gets a penalty equal to 1/2 the difference between the two stats, rounded up. For example, PC A has a PP of 20, and PC B has a PP of 15. There is a difference of 5, and half that (rounded up) would be 3. PC A rolls d20, while PC B rolls d20-3.
- In both of these instances, the higher roll wins. In these cases, a natural 1 is a failure, while a natural 20 is a success. If both players roll a natural 1 or both players roll a natural 20, they both roll again. You can't have both fail or both succeed when they are making an opposed roll.
Does this help?