IKEA Besta Manual page 27

Step 6
Insert a tapered head screw (#112522) into the hole in the side chair rail and start threads by hand.
Use the Allen wrench (#100001) to tighten the screw. Repeat for all 4 holes.
Step 7
Position the chair and backrest as shown above. Insert a tapered head screw (#112522) into the hole
in the side chair rail and start threads by hand. Use the Allen wrench (#100001) to tighten the screw.
Repeat this for both holes.