Laws of the Congregation's responding to the Kaddish (5 sections): One should have concentration when answering the Kaddish. Rem"a: And one does not interrupt between "Y'hei Sh'meih Rabbah" ("May the Great Name") and "M'varakh" ("be blessed") [Hagahot Asheri in the name of Or Zarua wrote regarding the first explanation: "One should not interrupt between "Sh'meih" and "Rabbah" and according to the explanation of the Ri: one should not interrupt between "Rabbah" and "M'varakh"]. and one should answer loudly and strive to run to hear Kaddish. Rem"a: And one should stand when answering Kaddish and any davar she-bikdushah ("matter in which there is holiness" - a recitation which requires a minyan). And one who comes to synagogue and hears the congregation answering Kaddish answers along with them even though he did not hear the prayer-leader saying "Yitgadal etc." [Hagahot Chadashim on the Mordechai on Brakhot]. And the prayer-leader also needs to say "Y'hei Sh'meih Rabbah...". And when he begins "Yitgadal...", one should say "And now, let power [of YHVH] be magnified..." (Numbers 14:17) and "Remember Your mercy..." (Psalms 25:6).
When the chazzan says: "yitbarakh" everyone should answer "amen", and so too when he says "b'rikh hu", and "v'imru amen". Rem"a: And the custom is to not say "amen" after "yitbarakh" and not after "b'rikh hu". And one may not interrupt between "b'rikh hu" and "l'eila mikol birkhata..."
Those who respond up until "l'almai almaya" only [and stop] - they err, for it is forbidden to interrupt between "almaya" and "yitbarakh".
When the chazzan says "yitgadal", he should bow, and similarly at "y'hei Sh'meih Rabbah", and similarly at "yitbarakh", and similarly at "b'rikh hu", and similarly at "amen".
After one is done saying kaddish, one steps 3 steps back and then says "oseh shalom...".