gradually

step by step, by degrees, steadily, increasingly, slowly, regularly, a little at a time, little by little, stone by stone, bit by bit, inch by inch, grade by grade, by installments, in small doses, continuously, in due succession, through all gradations, by regular stages, progressively, successively, sequentially, serially, constantly, unceasingly, perceptibly, imperceptibly, insinuatingly, deliberately.

Antonyms quickly*, haphazardly*, by leaps and bounds.