In Mincha on the fast of Tish’a B’Av, the prayer nachem is added to the beracha of v’liY’rushalayim, and the conclusion of the beracha is slightly changed.
If one forgets to say it, one should say it in the beracha of r’tzei (in the place of ya’aleh v’yovo), but the regular ending of that beracha is said.
If it was forgotten there as well, it should be said in the elokaiy, netzor prayer, but without the chasimah (the ending of the beracha).
Even a person who is not fasting should say nachem.
One who is eating should say nachem before the words u’v’naiy Yerushalayim in bentching.
The Chofetz Chaim writes that every single Jew participates in building the Beis HaMikdash when he or she treat the mikdash me’aht which we have (in the form of our Botei K’nesses and Botei Medrash ) with the respect and awe due it and its kedusha. And the converse….
So this Tish’a B’Av, commit yourself to not shmoozing, not joking around, not carrying on, in the Beit K’nesses/Beis Medrash, and thus participate in rebuilding the ultimate, real, Beis HaMikdash.