Can someone give me guide to get levtover

I am not sure but i think we need 10 Nuggets,which can be obtained from the map if we have equipped Itemfinder or Dowsing Machine in our Hands in the equip menu!
These nuggets can be researched at the laboratory for getting a leftover!

Iirc, the nuggets are for black sludge.
The item you needed is that ruby/sapphire.

black sludge what for?

Use item finder and Dowsing machine and search the maps and collect 50 rubies and 50 sapphires,then go to the lab and hand them in and it's a 50% chance of getting leftovers or collect 10 nuggets and it's the same 50% chance of getting black sludge.Black sludge protects you're pokemon against poison attacks.