You'll have to look at groceries that import Mediterranean food for grape leaves. Your goal is to find grape leaves that have very thin veins, which will make the taste less stringy: if you are in luck and the store sells them in glass jars, then be choosy; otherwise, pray for the best and, if unsatisfied with one brand, try another next time.
Boil 8 cups water in a large pot. Remove the leaves from the jar, and unroll them. There is no need to separate individual leaves yet; just unroll/unwrap the batch from the jar. Place the leaves in the pot, reduce the heat to medium, and cover the pot. When the water boils again, turn off the heat, and let the leaves sit in the hot water for 10 minutes. Remove the leaves from the pot. Empty out the pot, and replace the hot water with cold water from the tap. Put the leaves in the cold water and set aside.
The above boil-and-cool procedure removes the bitterness of the grape leaves. Some recipes, esp. those used for commercially packaged dolmadakia, do not include this step, thereby producing fairly bitter wraps. The boil-and-cool cycle can be repeated to reduce bitterness even more; but do it too many times and the leaves will turn to mush (like boiled spinach).
Brown them in
Move the browned onions in a bowl, and add:
Knead well the above mixture with your hands, and set it aside as the filling. This filling is the traditional one used for a main Greek course; an equally traditional version of dolmadakia served as appetizer uses pre-cooked rice and nuts, while commercially packaged dolmadakia use other creative fillings (curry, strict vegetarian, etc.). This is why the label dolmadakia is very generic; like sausage, whose taste varies depending on the meats used, people's like or dislike towards dolmadakia is very specific to their individual chef who prepare them. The above filling has only gotten top marks so far, even by people who had been turned off by the commercially packaged dolmadakia.
It is now time to roll the dolmadakia:
To protect the bottom layer of dolmadakia from the heat, place a plate at the bottom of the pot. Then, line the plate with all the grape leaves that you couldn't use (because they were too small, or torn, or because you ran out of filling).
Pack the dolmadakia as closely as possible: you want to make sure that they won't unwrap while cooking as the water boils and agitates the dolmadakia.
Place a second plate upside-down on top of the topmost layer of dolmadakia.
Finally, place something heavy (but clean) on top of the plate, such as a stone, an iron, a block of marble, a bowl filled with glass beads, extra plates, etc. Whatever you use as weight, make sure it allows the pot cover to securely seal the pot, thus trapping the steam.
Fill the pot with water up to 1/2 inch above the top plate, and let cook for 1.5 hours in the lowest setting that still allows the water to boil gently. You may have to add water while the dolmadakia are cooking since rice absorbs water. 1.5 hours is a guideline: depending on your heat setting, which may be too low until you get the hang of it, you may need to wait longer or increase the heat setting. What's best is to start with a conservatively low setting, wait 1.5 hours, and then taste a dolma: if the rice is cooked, you are done. If not, increase the heat a little bit and take another taste 15 minutes later; and next time you try this recipe, use this higher setting.
Turn off the heat, and let the pot cool for 4 hours. Remove the pot cover and weight. Empty the water out of the pot by tilting the pot over the sink, while pushing the top plate against the dolmadakia (so that they don't come out). Remove the top plate, and carefully move the dolmadakia one by one onto a serving plate.
Serve hot with yogurt on the side, which is excellent as a dolmadakia dip.