Measure around the widest part of the shoulder to decide which size to make.
If you want to make a size not listed, use the gauge swatch and schematic in the pattern to make the desired size.

The hexagons are joined as you go and the flowers are added after the pieces are blocked.

There are directions, diagrams, photo and video tutorials for joining to the different sides and how to make the flowers as well as the different sizes for the poncho.