postupati

1. to act; to proceed
   Policija postupa po zakonu. — The police act in accordance with the law.
   Uvek postupa brzo i bez razmišljanja. — He always acts quickly and without thinking.
2. (s + instr., prema + dat.) to treat; to handle
   Loše postupa sa svojim radnicima. — He treats his workers badly.
   Sa ovim hemikalijama treba pažljivo postupati. — These chemicals must be handled carefully.

In the sense "to treat" it takes s/sa + instrumental or prema + dative.