IKEA Besta Manual page 10

Step 10
A.
Attach the flat pieces with slots (#124168 - #124173). The longest pieces (#124168 and
124169) go all the way at the end with two on each side. Attach them to the clips by
positioning the pieces vertically with the hooks on the bottom and facing towards the
shelf. Push them into the clip and then rotate them down so that they cover up the clip
from the top.
B.
Place the smaller pieces (#124170 and 124171) just inside the ones you just placed
using the same method.
C.
The last piece with the lip (#124172 and 124173) goes directly in the center of the shelf.
It can just be placed flat the lip on the outside. The notch on the bottom should fit into
the clip.
D.
Make sure that each clip is spaced so that it is connected to two flat pieces on the ends.