When notating multiple flat notes in a single measure (e.g. B-flat, or "a semitone lower than B"), the symbol only needs to be applied to the first note in that measure.
In the following example, the third, fourth, sixth and seventh notes are "flat"; the fifth isn't affected by the previous measure, and the eighth note explicitly goes back to being a "natural B".
Flat (online notation software) is pretty cool. Apparently, you can even edit a score with other people, Google-docs-style. Highly recommended.