There is no secret recipe or right/wrong way to make it. I know there's a lot of different recipes for it, but this is how I make mine.
1 can of tuna, drained
potatoes, cooked (I usually eyeball, about 1 big potato for each of us)
half an onion, chopped
2 hard boiled eggs, sliced
I start by cooking my potatoes either in a pan on the stovetop, or for a quicker way, I do it in the microwave. Four potatoes for about 10 minutes or so, just keep an eye on them. Remember to poke some holes in them before.
In a serving bowl I put the tuna, then add the onions, potatoes (chopped into bite size pieces), lettuce and mayonnaise, combine everything and then add the hard boiled eggs.
This is actually a very filling meal. When I have fresh lettuce at hand, I add that in there too. Be creative, add whatever you wish :)