The lines in the cubes should be even and not collapse. Draw with a thin sharp pencil 0.3. If you draw with a rough pencil, there will be dirt and the lines are not accurate.

The proper way to draw straight lines is to use your whole arm and involve as few muscles as possible. One of the main reasons people struggle to draw straight lines is because they involve their fingers and wrist too much. Instead, when you use your arm to control where the line goes, the result is always a much more straight and steady line.
Draw lines without holding your pencil too hard. Don’t stress if they don’t come out perfectly right away. Remember this is just a practice. Keep practicing straight lines daily, and they will look fantastic in just a few weeks!