As for quantity, to dye the eggs, I take a gallon-sized plastic bag loosely stuffed with onion skins and put them in the pot with enough water to cover them while keeping them close to each other in the pan. I then add white vinegar, judging mainly by the smell of the vinegar mixed with the water. You should have a strong vinegar smell, but not one that send you for your handkerchief. This is approximately four tablespoons of vinegar to about three cups of water. I bring it all to a boil for a couple of minutes, stirring it as it boils, and then turn it down to a simmer. If you'd like a recipe with more precise measurements, try this Greek Easter Egg Dye recipe from Nancy Gaifyllia.