Start by assembling all of the food along with a knife, cutting board and large bowl.
Carefully dice the onions into small pieces. Put the pieces in the bowl. Sprinkle it with lime and salt; set aside.
Dice the tomatoes into bite size pieces and put them in the bowl with the onion, lime and salt.
Chop the cilantro and add it to the tomato/onion mixture.
Let your pico sit for a few minutes and then test it out to see if it needs any additional salt or lime. Keep in mind the flavors will become stronger the longer it sits so be careful not to over salt.