The Laws of the Blessing of the Years. Containing 5 S'ifim:
[Regarding] the Blessing of the Years: In the rainy season, one must say in [the blessing] - "And give dew and rain". And in the Diaspora we start to ask for rain in the evening prayer of the 60th day after the autumnal equinox (and the day of the equinox is included [as day one of the 60 day count]) (Hagahot Maimoni Chapter 2). And in the land of Israel we start to ask [for rain] from the night of 7 Marcheshvan and we [continue to] ask up until the afternoon prayer of the eve of the first Yom Tov of Pesach; and from then onwards, we stop asking.
The individuals who need rain in the hot season should not ask for it in the Blessing of the Years, but rather in [the blessing of] "Shomeya Tefilla" ("Who hears prayers"). And even a large city such as Nin'veh or one whole land such as S'pharad [Spain] in its entirety or Ashkenaz [Germany] in its entirety - they are considered as individuals [and should ask] in "Shomeya Tefilla". However, if [someone is] in one whole land where they require rain in the hot season erred regarding it and asked for rain in the Blessing of Years, (if one desires,) one goes back and prays according to the rules of voluntarily prayer without the request [for rain] in the Blessing of Years. (But one is not obligated to go back at all.) (Beit Yosef in the name of Maharia [R Israel Isserlein], and the Ramban and the Ran hold like the Rosh)
If one asked for rain in the hot season - we make [that person] go back [and pray again].
If one didn't ask for rain in the rainy season, we make [that person] go back [and pray again] even though [that person] asked for dew. But if [that person] asked for rain and not dew, we do not make [that person] go back [and pray again].
If one did not ask for rain and remembered prior to [the blessing of] "Shomeya Tefilla" ["Who hears prayers", the last of the middle, petitionary, blessings] we do not make [that person] go back, and one may [instead] ask in "Shomeya Tefilla". (And if one [took upon oneself] a fast and must say "Aneinu" ["Answer Us", the insertion for an individual], one says the question [i.e. asks for rain] before "Aneinu.") (Abudraham) And if one does not remember until after "Shomeya Tefilla" - if one has not yet moved one's feet [i.e. taken one's 3 steps back at the end of the Amidah], one goes back to the Blessing of Years; and if one has moved one's feet, one goes back to the beginning of the prayer. And if one completed one's prayer [i.e. Amidah] and one is not accustomed to say supplications after one's prayer, even though one has not yet moved one's feet, they are [considered] as if they were moved. And if one remembered after one concluded [the blessing of] "Shomeya Tefilla" but prior to starting [the blessing of] of "Retzei" ("Be pleased"), it seems that one should say "And give dew and rain" [right then and there] and afterwards says "Retzei".