Ask-a-Dave's-Gardener: Planting Violas: Soil Depth , 1 by Angelickho

Angelickho wrote:
If it is a viola you purchased from garden centre just plant them at the same depth as that on their cell pack. Make sure you plant it on a bigger pot than that of the cell pack. Mounding violas should be spaced about 6-8 inches apart. Trailing or spreading varieties can be planted 10-12 inches apart.

If it is seed ( germinate indoors )

Good well-draining soil is recommended ( i usually add a handful of washed wet Vermiculite on my soil John Innes Seedling Soil ). Make sure your potting mix is moisten before you add the seed. Sprinkle the viola seed.
Cover with about 1/4 soil. If you plant it on pot or seed tray, i wrapped the pot/ tray with a cling film then put it on a black bin bag as Viola need darkness to germinate, so cover the seed completely. Some cover it with a newspaper or cardboard.
Keep the seed bed moist. Set in warm location ( 18- 21 degrees C ) - i put mine on airing boiler cupboard or on top of the fridge. Seeds should begin to germinate in 7 - 10 days.
As seedlings develop, thin plants to about 6- 8 inches apart. Transplant excess seedlings.