I found an interesting diagram drawn by one Kaytee posted in the website of the National Cockatiel Society (NCS).
According to Nancy Kizuka of NCS Magazine, the bird's wings need to be clipped again after the next molt. She says, "When clipping these flight feathers, be careful not to clip any growing feathers. You will recognize these blood feathers from the others since you can see the dark blood area in the shaft."
Kizuka also wrote something about clipping the nails of the bird.
"Nails need to be clipped when they are becoming snagged on toys, cage covers or clothes. One can use a human nail clipper or baby nail scissors, and take off just the tippy-tips.
All nails have a blood supply. If you nick into this blood supply or quick, have some Quik-Stop or Stay on hand. If you do not have these commercial products on hand, flour or corn starch will work. Apply this with gentle pressure until the bleeding stops. Occasionally the nail gets cut too short and these products don't work. Silver nitrate sticks stop any bleeding. They are available from your avian veterinarian.
The proper size perch can help keep nails at the correct length. The tips of the nails should touch the perch. You can also use concrete perches, pedi-perches, sandy perches, hard wood perches or any of the other conditioning perches available today. Using a variety of perches also helps the bird stay comfortable since they are on their feet 24 hours a day, 7 days a week. It also helps to prevent early onset arthritis." (Source: NCS Magazine)
I've been clipping Pepper's nails and it has worked like a charm. He's much more balanced and gets a better grip of the perch. He also doesn't hurt himself anymore when he scratches himself.
