How to roll 100:

take 10 sided die, roll once

take that number and multiply by 10

roll 10 sided die again

add that to the number from #2

add 1 (otherwise it would be 0-99, not 1-100)

How do you get “1” like this?

I suppose you have to roll 10 to get to 100 = 0

But then what?

Modify step 2 so you subtract 1 from the roll first, then multiply by 10. That way if you roll a 1, the 10’s digit becomes a 0.