To me it, there is an easy to remember system to use depending on what dice you have available.
I think the problem stems from the fact that ten sided dice come either as 0-9 or 1-10 or 00-90.
From this you can potentially come up with six different combinations of these three different ten sided dice. But most people use one of three of these combinations to generate a number between 1 and 100:
- 0-9 and 00-90
The result of the 00-90 dice represents the tens place and the 0-9 represents the ones place. The only exception to this rule is if you roll 00 and 0, which represents 100.
00 and 9 is 9
50 and 0 is 50
00 and 0 is 100
- 1-10 and 00-90
Simply add the two dice results together.
00 and 1 is 1 10
10 and 10 is 20 90
90 and 10 is 100
- 0-9 and 0-9
This one is covered in the PHB. Roll one dice, which represents tens place, roll the other, that represents the ones place. Double 0 is 100.
0 and 8 is 8 5
5 and 3 is 53 0
0 and 0 is 100
The other dice combinations get a bit more confusing. I would stick to one of these three.