I chose the Bells pattern (which I had previously adapted from a 'normal' knitted trim pattern).
I chose to use John Arbon's Knit by Numbers double knitting yarn, which knits up at 22 st/10cm on 4mm needles. I used the pick up stitches page of the interactive spacing tables to figure out my multiplier (22 st/10cm ÷ 60 st/10cm = 0.37) and look at the potential spacing options. The nearest option was 0.38 (ratio 8:3, spacing 3, 3, 2) but I decided to jump to 0.40 (ratio 5:2, spacing 3, 2) to pack in more frills and make the picking up more straightforward.
I put the jumper on and basted a guide line for the frill position. Technically, it is a curve but appears level on the body.
Having knitted it, I tried on the jumper again. I liked the position of the frill, but decided that I'd packed in too many 'bells', and that the frill should stop before the seam. I undid it, and had another go, now picking up every 3 stitches. This time, I was happy - the frills were full enough without splaying out the foundation fabric.
I decided I wanted another frill, and wanted it to be in line. Using the first frill as a guide, I stitched a second guide line:
The guide helped me to get the second frill in line with the first:
I sewed the sides of the trim down, tidied away the ends and my frills were complete: