IKEA Besta Manual page 7

Step 4
A. Attach two more vertical shelf
pieces on the remaining two sets
of screws in the base,
B. Screw in two large
horizontal screws (part #114926
115263) into each piece as
shown to the left.
Step 5
Now take the top piece of the shelf and
screw in the 8 remaining extended head
screws (part #114928) from the top side
as shown to the left.