At Maariv on Tuesday night Dec. 4th, in Chutz L’eretz we begin to include Vesain Tal Umatar into the Shemonah Esrei in the beracha of “Boraich Aleinu”. (In Eretz Yisroel they have already begun on the 7th of Cheshvan). If one became aware, after one completed the Beracha of Mevaraich Hashanim, that one omitted Ve’sain Tal Umatar, one should wait to insert it right before Ki Ata Shomeah of Shema Koleinu. If one already completed the beracha of Shomeah Tfilah, one may insert it before saying “Retzei”. If one already began “Retzei” one must return to the bracha of “Boraich Aleinu”, which is the proper place for Vesain Tal Umatar. If one already completed the Shemonah Esrei and stepped backward, then one must repeat the entire Shemonah Esrei. Shulchan Aruch with Mishna Brurah 117:5 see also Bi’ur Halacha (It is advisable to repeat 101 times (Chasam Sofer OC:20) [at the very least 90 times]: Ve’es Col Minei S’vuasa Le’tova Ve’saen Tal Umatar so as to make the inclusion of Ve’sain Tal Umatar habitual and fluent, thus eliminating any future doubt as to whether one included Ve’sain Tal Umatar in the Shemonah Esrei or not.)