Years ago borders were laid out with high plants at the back, medium plants in the middle and short plants at the front. This is still good practice so that tall plants don't block light to shorter plants. However it does look a bit dated these days to have everything lined up regimentally in 3 levels. A bit like a Primary school photo. A more modern approach to layering plants is too use two exciting categories of plants with the power to mix things up. These are 'Tall & Airy' & 'Spikes and Spires' which can be planted front, middle and back.
Tall and Airy - as the name suggests grow very tall but have small 'button' flowers that don't cast any shade on surrounding smaller plants.
Spikes and Spires are tall and thin and don't cast much shade.
Repeating these groups of plants throughout a border in front, middle and back positions creates so much rhythm and interest. Invaluable plants!
For Shadier bits - use the same principles