This works with any kind of pepper – hot or sweet. In the images below, I’m slicing a jalapeño.
First, slice off the stem end so you can stand the pepper up, like this:
Holding the tip of the pepper (provided you're not holding a camera in addition to the knife) with one hand, slice down along the edge of the pepper, making the slice just thick enough to separate the flesh from the seeds. It takes a bit of guesswork, because some peppers are thicker than others, but with practice, you'll get the hang of it.
Once you've made the first cut, the rest are easier - you can see how thick the pepper is and follow it down with the knife as you make the next several cuts. Keep the knife in between the flesh and the seeds as you go...
And after a few slices, you're done! Discard the core with the seeds, trim the rib - the white part inside the pepper that connects the flesh to the core - and you're good to go.
Now dice the peppers and you're all set.
You probably know that handling hot peppers of any kind with bare hands can cause you some pain. The capsaicin in the pepper will get on your skin and seep into your pores and linger, so either wear gloves of some kind while you're chopping them, or wash your hands really well after you're done and avoid touching your own (or anyone else's) eyes or nostrils or other sensitive parts.
I don't wear gloves. Maybe it's foolish, but I just never think of it. I do, however, make sure to handle the peppers with only one hand. I use my left hand (I'm right-handed) to handle the pepper and my right hand to use the knife (and the camera - you should see the contortions I go through for "action shots" sometimes). That way, at least it's only one hand that burns.
FYI, I still felt the burn the next morning with these. And it's funny - when my hand was dry, I didn't notice it, but if I was washing dishes or something, the burning feeling would flare up. Same thing if I was holding something hot.
Eventually, though, the hot feeling went away. It wasn’t incapacitating…just kind of interesting.