How to seed a chile pepper
Most of the heat in a chile pepper -- whether jalapeño, serrano, habanero, or another -- comes from the white ribs and seeds. Removing both reduces the chile's heat and allows more of the pepper's flavor to come through.
First, cut the pepper in half lengthwise. If you want the halves to stay intact for larger slices or a large dice, just trim out the seeds and ribs, cut out the stem, and slice the pepper as thick or thin as you like.
Otherwise, it's easier to get the seeds and ribs out if you cut each half in half again.
To get the seeds and ribs out, hold each quarter by the end. With the knife parallel to the cutting board, slice across the ribs and seeds. When you get to the top of the pepper, turn the knife downward toward the cutting board and chop off the stem with the same motion. (Here, we used a Granton Santoku knife; you may prefer a smaller paring knife.)