Lemon/Lime amt can vary, simply taste as you add the juices!
Finely chop the onions, cilantro and tomatoes and add to a mixing bowl, then begin to add the oil, vinegar, lime/lemon juice and remember to salt and pepper as you go along!
Peel and devein your shrimp, chop and add to the bowl! Let the mix marinate in the lime/lemon oil/vinegar overnight for a delicious chilled ceviche! Eat with tortilla chips, preferably home made!