Hi!
Example: roll 1d4 - 1d4, the result generate your ability modifier.
Presence 1d4 (3) - 1d4 (2) = 1
1 is your modifier to all Presence rolls. To tests your roll d20 and put your modifier.
Example of roll test of Presence: 1d20 (15) + 1 = 16
General difficult rating of tests is 12. Equal or greater in your d20 (+/- your modifier) roll = success