I was always taught that you divide plants either in the fall after they've finished blooming or in the spring before they bloom.

Hi, Carol.

Thank you. Again, I have no idea what I'm doing.

Any idea of how to start periwinkle for transplanting?

Periwinkle is easy. Just cut off a piece and stick in water it will root.
I have it all over here. I inherited it when I moved into this house.
It grows easily. I have to trim mine since it edges the driveway in some spots. Whenever I trim it I take the trimmings and stick them in water. Once they root I simply plant them.
Another way is to take the cuttings and lay them so the cut tips are in a composted/dirt mixture and then keep damp till they root.
I think sticking it in water is much easier.

Carol, thank you so much - I have a huge area to plant and already have periwinkle growing (in the lawn no less). The garden book I have suggested 198 clumps for a 10 X 10 area... I'll need something like 600-700 clumps to get this area started.

Periwinkle likes damp compost rich soil...not wet. You'd be surprise how fast it will fill in. When we moved in here the patch near the drive was kind of sad looking. Some serious weeding and a few years of it getting some care and it's thick and rich looking.