I recommend using packets of tuna in water for absolute ease so there's nothing drain. Use two packets if they're about 2.6-ounces each. Or use a can of well-drained tuna packed in water. Most cans are 5 ounces, so use 1 can. I am not a fan of tuna packed in oil, but if you are, you can use it as long as it's drained. It's rare to have fresh leftover tuna (from a whole fish that you cooked the day before) on hand but if you do, flake it, make sure to carefully check for bones, and you can use it in the pasta salad.