Ok, more tinkering.
Reading around the web, there are lots of hints about using these feet. I tried the method in threadsmagazine I linked above, and I found it more difficult and less successful than what I was doing previously. so I went back to that.
1. Place material under the foot with the edge lined up where it would normally run, and sew a few stitches. Remove material and trim the thread long 6".
2. Using the long threads, guide material into and through the foot. The threads are tugging on the head of the material, making it easier to draw it through the foot.
3. When you have it looking good, drop the foot and start sewing. Its a bit of a guess to know how much material to allow into the foot, and managing that and keeping the stitch straight is not easy, but I have gotten better with practice.
Here is some of my better stitches:
and here is an area where I ran too lean on material in the foot and then corrected:
The other very worthwhile hint I found was to use a rotary cutter to trim the edges of rip stop. This is a brilliant idea, using a 45mm rotary cutter with a craft cutting board and a heavy straight-edge gave really smooth edges and definitely improved my result. I have a Fiskars rotary paper trimmer 45mm, and it does a great job on the rip stop: