If i look at this site, go to: uniforms/coat... you might get an idea.
To me the packet flap is clearly sewn on top of and slightly above the triangular slit.
I would say: hold the flap upside down and inside out, slightly above the slit then stichs throw flap and coat front panel.
Maybe in two rows of stiches for strength then fold the flap down. If everything is ok the flap would then cover the slit with the front of the flap foreward.... on some pictures on this site it also lookes as if the have been sewing also throw the top of the flap flattening the fold a bit...
But hey I am an beginner myself you know...
I'm shure there are more experiensed people around here...