Roll one die. 1 to 4 means you add, 5 or 6 means you subtract.
Roll one die again. This tells you the number (1 to 6) that you add or subtract.
Roll one die a third time. This tells you which of your attributes the number is permanently added to or subtracted from.
1=Strength
2=Intelligence
3=Luck
4=Constitution
5=Dexterity
6=Charisma
Example: you roll 1, 2, 4. That means you add 2 to your Constitution. Now go back.