inch by inch
In very small increments; very gradually. Can be used both literally and figuratively. It took us years, but we dug this tunnel inch by inch. You won't improve your grade all at once, but rather inch by inch throughout the semester.
inch by inch
Fig. one inch at a time; little by little. Traffic moved along inch by inch. Inch by inch, the snail moved across the stone.
ˌinch by ˈinch
very slowly and with great care or difficulty: She crawled forward inch by inch. inch by inch
Very gradually or slowly.