Once you sort out how to level then things are easy. 
1. Tighten legs
2. Loosely screw chair to back.
3. Place on table and press on seat till no wobbles.
4. Tighten upper middle bolt
5. Tighten other 2 top bolts.
6. Tighten bottom 2 bolts but don't do it as tight as uppers.
Job done.
N.B. the bolts for legs are longer than those for seat. Make sure you use correct ones.
Cheers
John