fantastic furniture TOKYO Assembly Instructions Manual page 6

Step 7A
Place the slat in position then push down on the
center of the slat to flatten out the slat. Make sure the
plastic prong lines up with the hole. Then push firmly.
Then push down on the center of the Long Plastic Connector
Repeat the same process for the other end of
the slat.
Step 7C
Step 7B
Page 5