If a person finishes p’sukei d’zimrah before the sh’li’ach tzibbur is ready to say yishtabach, the person should say yishtabach immediately and then wait for the sh’li’ach tzibbur to say it.
This is true even on Shabbos or Yom Tov–the person should conclude nishmas all the way until after yishtabach, and then wait. But this is true only if the person finished p’sukei d’zimrah well before the sh’li’ach tzibbur is up to there–if it is only a short wait, then he should of course say nishmas, and the following paragraphs until yishtabach, together with the whole tzibbur.
How many times does the Torah reiterate at the actual ‘putting together’ of the Mishkan “as HaShem commanded Moshe”? (Go ahead–count ’em!) This may refer to B’nei Yisrael’s enthusiasm, their cognizance of the kedushas Mishkan despite the many months that had gone by since the original command was given. Our appreciation and our emotional reverence towards kedushas Mishkan, Mikdash, and Beis K’nesses should never be allowed to dim.