Pin one is marked on that diagram as being the pin at the bottom left of the display as you look at it from the front.
The rest will be numbered in the same way as DIP IC's usually are, counting along on side, and then back along the other.
So, looking at the display from the front:
12 11 10 9 8 7
o o o o o o
o o o o o o
1 2 3 4 5 6