The position of the clamps can be seen after the first pull.  The first main string has been tensioned in the below photo:

The first three crosses are prestrung and the starting clamp is in place. This situation is now ready to pull the first cross:

 

Now the first cross is pulled:

Now the second cross is pulled, the clamps in place, and ready to pull the top cross before tying the top knot (first of two knots for the crosses) on a two piece string job: