If you forget to say v’sain tal u’mattar liveracha (which starts this Thursday night!), if the Name of HaShem at the conclusion of the berachah was not yet said, go back to v’sain… and say from there.
If the Name of HaShem was mentioned already, continue, and insert v’sain… in the middle of shema koleinu.
If forgotten there, and remembered right after the Name of HaShem was said at the conclusion of shomei’a tefillah, say lamdeinu chukechoh, insert the phrase v’sain… and continue with ki attah shome’ah….
To be continued…