On a dry stone bridge the voussoirs are shaped as close to parallel bedded trapezoids as possible and then laid in rows optimally with their best face down and their longest length up. This first 'grocery bag' size/shaped voussoir is being pinned from the back
The closer that voussoirs can fit together at their faces ( the exposed intrude side) the better.
Avoid using stones for voussoirs that are more like triangles than squares (that is, ones that get too narrow towards their upper end when placed face down on the form)
If their sides don't fit well and lock, as they butted together along the row, try other combinations of voussoirs until they do. Each odd shaped voussoir should lock in somehow to the other or at least overlap on one side to the one next to it.
It is best if the stones in a row of voussoirs are all the same thickness so that the next stones can be laid across the joints. Sometimes a pair of voussoirs can be laid together to make up the height of the other stones in a row of voussoirs.
These arch stones can be pinned with thin wedge stones as they are put in so that each one fits fairly snug in the structure as it is being built. Aviod letting any wedges slip down and thus create pivot points between stones. The pins should be near the top and only be used to allow the faces of the voussoirs to touch, rather than be a place where the stones rock against each other.
From the side looking across the bridge the voussoirs should all be set so that they fan out slightly along the lines drawn on the form. These wedges are pushed in just far enough so that the voussoirs keep their orientation. The final 'shimming' which locks things permanently tight will be done when all the voussoirs in the entire arch have been set properly in place.
Achieve the proper angle along the rows of voussoirs by pinning each voussoir (therefore effectively wedging it) is quite structural and can be done instead of having to shape every voussoir. By this method many useful, more random shaped stones, that have little or no taper, can be used. A more informal rustic look is created if enough of the stones in the bridge are pinned.
Shaping voussoirs, by sawing and chiseling every single one along their length to give them the slight taper required to fit in a radiating pattern (so that they are absolutely flush with the others) can become an unnecessarily time consuming job.
More on this point tomorrow...