Writing test practice
At the interview the officer reads a sentence aloud and you write what you hear. Listen, write it, then compare.
How the real test works
The officer reads up to three sentences and you must write one correctly. Spelling, capitalization, and punctuation mistakes are allowed if they don't change the meaning; you can't leave out a word that changes it. One correct sentence passes. Practice sentences here use only the words on the official USCIS writing vocabulary list; the actual test sentences are not published, so these are practice sentences, not official ones.
Five at a time
Each session is five sentences. Sentences you mark for practice come back at the end of the session. When you can get through a session without marking any, move to the reading practice, then the N-400 interview practice, which is where the speaking part of the English test happens.
The vocabulary list and more about the test: the writing test page.