You count as if you have 2 symbols instead of 10.
So we count 0, 1. Oops, we ran out of symbols, time to start a new digit!
10, 11. Ran out, start again!
100, 101, 110, 111. Ran out, start again!
1000, 1001, 1010, 1011, 1100, 1101, 1110, 1111. Ran out, start again. And so on, and so forth...
If you didn't get it yet, here's a small list.
0=0
1=1
2=10
3=11
4=100
5=101
6=110
7=111
8=1000
9=1001
10=1010
Make sense?
There are 10 types of people in this world, those who understand binary, and those who don't.