Are dogwood flowers even edible? Be careful not to pick something poisonous...
Anyway, my winemaking book (1958, H.E. Bravery) suggests using 3 pounds white sugar and one teaspoon nutrients to one gallon of water. Boil the sugar water to pasteurize it, add nutrients at flameout, let it cool and pitch a yeast starter of champagne yeast.
Ferment 48 hours and add elderflowers (2 pints to a gallon, and JUST the flowers, as I think elderberries have unpleasant stuff in the stems/leaves), or rose petals (3 pints/gallon), to make flower-scented wines.
I hope that helps somehow... at least as starting guidelines. Someone more experienced could also probably help you, and a man named Jack Keller has a website devoted to funky wines of this nature, too.