Line up up the side panels with the back coat and pin into place (the longer seam allowance on the side panel is sewn along the back seam, fyi). Sew along the length of each side panel.
The bottom hem straightens out a bit -make sure your sewing line straightens out a bit as well.
Press open your seam allowances.
Pin your coat front to the right side panel. The flat part of the side panel seam should line up with the corner jutting out of the front coat seam. Line up your notches; note that one of those notches should align with your dart.
Sew along the length of the seam and press it open.
From the outside it will look something like this.
The seam allowances will stick out a little along the armscye - trim them with scissors to create a nice smooth curve.
And that's it for today - sleeves on Monday!