* May we never have to do so
One should tear Kriyah:
The tear should be made on one's upper most garment, it should be one tefach in length (around 3.5-4 inches or around 9cm).
One must tear kriyah standing.
When tearing kriyah for a relative one should make the following Bracha before tearing: "Baruch ata Adonai Eloheinu Melech Ha'olam Dayan Ha'Emet - You are the Source of all blessing, God, our Master and Source of our abilities, King of the Universe, you are The True Judge."
When tearing kriyah for Har Ha'Bayit one should say the following before tearing: "Beit Kodsheinu V'Tifarteinu Asher Hillelucha Bo Avoteinu Haya L'Sreifat Esh, V'Chol Machmadeinu Haya L'Charava - Our house of holiness and glory, in which our forefathers sang praise to you, has been burnt, and all that we hold dear and precious has been destroyed"
One should also tear Kriyah when seeing Jerusalem or other Biblical cities in a state of destruction (according to some poskim even hearing news of their destruction) and say the following: (For Jerusalem) "Zion Midbar Hayta - Tzion has become a desolate wilderness" (For Destroyed Biblical Cities) "Arei Kadshecha hayu midbar - Your Holy Cities have become a desolate wilderness" If should, however, be noted that many poskim hold that Jerusalem is not considered destroyed today - because it is in our hands - and one would therefore not tear upon seeing it. Consult your local Halachic authority for more on this.
May we know of only smachot and never of sorrow.
(Halachot of Tearing Kriyah for a relative can be found in Shulchan Aruch Yoreh Deah 340:1-4 and for tearing on Har Ha'Bayit in Shulchan Aruch Orach Chaim 561:12)