Use the phrase "little by little" to describe something that happens slowly. Another way to say this is "gradually", but "gradually" is more formal. "Little by little" can be used for both positive and negative things:

We're starting to get the hang of it little by little.

She's losing her memory little by little. Soon we're going to have to move her into a nursing home.

There are actually a lot of phrases like this that express doing something slowly and steadily. Here are a few:

bit by bit

inch by inch

day by day

one by one