IKEA Besta Manual page 6

Step 2
Turn the base piece back upright, and place the 8
screws (part #113287) into the 8 holes positioned along
the long sides of the base piece.
NOTE: These are positioned on both long sides at both
ends and 1/3 and 2/3 of the length (4 per side as shown
to the left).
Step 3
A. Attach both vertical end pieces to the
outside sets of screws from Step 2.
B. Attach 4 small horizontal screw heads
(part #114927 115624) into the holes at
the base of the vertical end pieces, 2
per side as shown to the left.