Term for doing something little by little?

Incrementally

Incremental, according to Merriam Webster:

of, relating to, being, or occurring in especially small increments [e.g.] incremental additions

Vocabulary.com says:

If you are making incremental progress in math, you are moving slowly but steadily forward. Incremental describes regular, measurable movements that are usually small

Incrementally is doing things by increments. In your case you are reviewing your course-work incrementally, in easy to absorb bite-size portions.

Doing this for a week is better in the long run than cramming for hours the night before an exam; you will retain more.

The review for the exam was 'piecemeal' (adv. and adj.):

1. By a small amount at a time; in stages: articles acquired piecemeal.

[piecemeal. (n.d.) American Heritage® Dictionary of the English Language, Fifth Edition. (2011). Retrieved January 20 2016 from http://www.thefreedictionary.com/piecemeal . Emphasis mine.]

For example,

Her review for the exam was piecemeal.

'Piecemeal' communicates the chronological dimension of what I understand to be the meaning you want.

Pace

Google Definition:

Verb

do something at a slow and steady rate or speed in order to avoid overexerting oneself.

Rather than cramming for an exam, a student should pace her studies throughout the semester.

Update in response to downvote.

While the definition of the word does not exactly fit the OP's word "little by little," I feel this word is very commonly used in the above situation by students:

Hey, what are you trying to do? Memorize the entire book in the first week of class? Pace yourself!

(while cramming) I'm not going to put myself through this again. I'm going to pace myself next semester.

Iteratively

b : relating to or being iteration of an operation or procedure

So then, what's iteration?

1 : the action or a process of iterating or repeating: as
a : a procedure in which repetition of a sequence of operations yields results successively closer to a desired result