Outside of Eretz Yisrael, begin saying Tein tal u'matar l'vracha at ma'ariv of Dec. 4. In a secular leap year, begin saying it at ma'ariv of Dec. 5 (but there might be rare exceptions!).

Where: Amida 9^th paragraph
Error:  Said tein bracha or tein tal u'matar l'vracha in the incorrect season
Situation Did Not Finish Paragraph
What to Do You must return to beginning of paragraph (bareich aleinu).

Situation Already Finished that Paragraph
What to Do

• If you said tein bracha in the wrong season, you may add tein tal u'matar l'vracha  in shema koleinu, just before ki ata shomei'a...
• If you said tein tal u'matar l'vracha in the wrong season, you must return to the top of the paragraph.

Situation Already Finished Shema Koleinu
What to Do If you did not correct your mistake in shema koleinu, you must return to the beginning of bareich aleinu.

Situation Already Finished Amida (such as you are ready to take 3 steps backward when you realize your error.)
What to Do You must repeat the entire amida.
Note If you said tein tal u'matar l'vracha in the wrong season but you are in a place that needs rain, you do not need to correct yourself or repeat that blessing.