Cut open the jalapeños (wear rubber gloves or put a small ziploc bag on your hand while you cut). Dig out the seeds and dispose. Dice the jalapeño.
Dice the onion. (*Optional=soak the onions in a bowl of water with a Tbsp. of salt for at least 30 minutes. Drain and rinse well. This takes away the bite of the raw onion and is easier on sensitive stomachs.)