It’s very common for Self assembly dining chair to wobbling slightly after assembled. It’s because of the legs assembled in different length on levelling.
The most simple trick to fix it as below step:
1) Unloose slightly for all screw and fitting which underneath the chair structure.
2) Once unloose the screw, put heavier thing or body weight on the chair seat to force it into same level on each chair legs.
3) Check the levelling again and make sure all legs are even or same leveling now.
4) Tighten the screw again。
5） The chair will become stable and no wobbling anymore.