Fit as many cucumber spears as possible into a 1-quart mason jar along with the dill, garlic, mustard, salt and sugar. Combine the vinegar with 2 cups water in a small saucepan and bring to a simmer. Pour the hot mixture into the jar with the cucumbers (you may have some left over) and swirl gently to combine (be careful; the jar will be hot). Let cool to room temperature, then cover and refrigerate at least overnight and preferably 2 days. The pickles keep, refrigerated, up to 2 weeks.