IKEA Besta Manual page 8

Step 6
A. Take the top piece of the shelf and
attach it to the rest of the shelf,
leaving a small gap along the back
side as shown above on the left.
B. Take your 4 remaining large
horizontal screws(part #114926
115263) and attach them to the top
side of the 2 inside vertical pieces.
Step 7
A. Insert the small horizontal screws
(#114927) into the top holes on
either end of the shelf.
B. Put the screw in with the small
arrow pointing up.
C. Use a screwdriver to screw it in until
the arrow points down.