A piece can be broken down into its component parts in at least two ways: vertically and horizontally.
Breaking down a piece “VERTICALLY” means to break it into sections--to work on single measures, two-measure segments, phrases, pairs of phrases, single pages, pairs of pages, or any variation or combination of the above. If the lines you draw are basically vertical (see green lines), you are "breaking it down vertically."
When deciding the size of the sections, consider carefully these two factors:
If the sections are too large, progress could be slow enough that motivation could dwindle.