1. One thing at a time
   "They approached the complex project in a step-by-step manner";
   - bit-by-bit, in small stages, piecemeal, stepwise

1. In a gradual manner
   "They explained the process step by step";
   - gradually, bit by bit
    
2. Proceeding in steps
   "the voltage was increased step by step";
   - stepwise

See also: gradual

Encyclopedia: Step-by-step