Brush through your hair to smooth it. If you have bald spots, brush gently over the hair around these areas, making sure your hair is free of knots and tangles before inserting the hair extensions.
Leave a small section of hair down at the bottom of the back of your head, spanning from about the bottom of one ear to the other. Clip the rest of your hair out of the way on top of your head. You should have a clean part spanning horizontally across the back of your head. This is where you'll place your first extension -- the second largest piece.
Line up the center of the hair extension with the middle of the back of your head, positioning it just below the part. Attach the center clip or clips onto the hair. Now attach the sides. The larger extensions usually have four to six clips, so you should be able to clip the extension onto hair on either side of a bald spot, even if you have to leave a few clips closed over the spots themselves.
Let down another layer of your natural hair -- just enough to cover the clips on the extension. This is the section you'll add your next extension to. Clip the rest of your hair out of the way.
Continue adding extensions in this manner working from the bottom up on your head, alternating between the extension pieces and your natural hair. If you have bald spots near the top of your head, they can be trickier to deal with, because you want to avoid having the clips on the extensions show through. To cover a bald spot that is high on your head, attach a single-clip extension over each spot, and tease the hair over top of it to create volume and cover the clip. Use a mirror to check that everything's in place and, voila -- perfection!
If you have a lot of bald spots and are missing quite a bit of hair, try using extensions in two different shades: one slightly lighter than the other. Use both sets of extensions, using most of the lighter pieces around the top area. This adds volume and creates definition in your hair, and gives the appearance of a full, thick head of hair.
Bald spots can easily become red and irritated, so avoid clipping the extensions too close around the perimeter of any spots.