At Maariv tonight, December 4th, residents of chutz la’aretz begin to say Vesein Tal Umatar during the Shemonah Esrei in the bracha of Boreich Aleinu. Residents of Eretz Yisroel already began saying Vesein Tal Umatar on the 7th of Cheshvan.
If one became aware after completing the bracha of Mevareich Hashanim that one omitted Vesein Tal Umatar, one should wait to insert it right before “Ki atah shomeiah” in Shema Koleinu.
If one has already completed the bracha of Shomeiah Tefillah, one may insert Vesein Tal Umatar before saying Retzei. If one has already started Retzei, one must return to the bracha of Boreich Aleinu, which is the proper place for Vesein Tal Umatar. If one already completed the Shemonah Esrei and stepped backward, one must repeat the entire Shemonah Esrei (Shulchan Aruch with Mishna Berurah 117:5. See also Bi’ur Halacha).
(It is advisable to repeat 101 times [and at the very least 90 times] “Ve’es kol minei sevu’asah letovah vesein tal umatar” so as to make the inclusion of Vesein Tal Umatar habitual and fluent, thus eliminating any future doubt as to whether one included Vesein Tal Umatar in Shemonah Esrei or not.