Right dbailly, very important that flashing as much as the joinery or fastening.
Be aware also that Code requires the structure and construction of lower roof (and wall) to be able to carry its dead load and snow load, plus the added snow load of the whole roof area above it (that side of the orig TF House). That's assuming preparation for worst case if all the snow on upper roof slides down and stays on lower pitched lower roof.
With good planning you won't need good luck. Post some pics when you get going w this.
Steve