Raise your guide lines to the height of the first course -- or layer -- which should be about the average size of your thickest, biggest stones. Hold back the most attractive, uniform stones for the top of the wall, but it's more important to get rid of the heaviest stones first so you needn't lift them later, even if they're beautiful. Lay them out in parallel lines along both outside edges of your trench, then fill in the middle spaces with less perfect stones -- if you can, try to preserve that internal V slope, especially here at the beginning.
You're looking for a flattish outside surface as well as -- more importantly -- preserving topside flatness for the stones to come. If you've chosen a dry-stone technique, of course, you have already made some aesthetic choices here, so pay attention to the outer-vertical (the "face") only insofar as you don't get any strange gravitational effects by having the stones stick out too far in odd places. Either way, you're going to want to strive for flatness for each course.
Make sure the stones are as level and stable as possible as you work. It's always better to knock off a wobbly bit from a stone -- or dig out a chip in the one beneath -- than to shim them in with smaller rocks or wedges. Save those for the interior of the wall, because the pebble you shove in to fix a momentary problem is the first thing that's going to fall out once you walk away. (And once you've filled in the first course as well as you can, for a mortared wall you cover the whole course anyway.) If you've got a rock that would be great, except for one feature or problem it's giving you, the best way to split it off is to score around that troublesome bit (a half-inch or less) with a chisel before you try knocking if off.
Finally, every 6 feet (1.8 meters), you'll want a "tie" stone, which is one that you lay crosswise rather than along the parallel sides. This literally ties the wall together, pressing downward on the stones below in a way that brings the two sides together into a single wall, and the tie stones become more important as you build upwards. Two perfect halves of a wall, no matter how cunningly you fill in the middle, are not what you're aiming toward -- the tie stones are the key to the wall's strength and endurance.