bit-by-bit

Also, little by little. Gradually, by small degrees, slowly. For example, The squirrels dug up the lawn bit by bit, till we had almost no grass, or Little by little he began to understand what John was getting at. The first term was first recorded in 1849, although bit in the sense of "small amount" is much older; the variant dates from the 1400s.

The adjective has one meaning:

Meaning #1: one thing at a time
  Synonyms: in small stages, piecemeal, step-by-step, stepwise