This underarm gusset design detail is one I found in the Japanese sewing book that I reviewed in an earlier blog. I had a bit of trouble sewing it in the first time but I left it in and hemmed the sleeve that way. It came out a bit wonky, but I still think this design feature is very unexpected and an indication of good dressmaking.
There are places for matchpoints in the gusset and on the side seams, which need to be marked.
|The bodice and gusset pattern pieces|
|This is what you end up with|
Then I staystitch with a small stitch length along the line, and clip right to the stitching (don't cut the thread) several times so that the edge can be opened out straight.
If you did an accurate job cutting the gusset pieces out, you should come out even at the sleeve edge. (Not like I did my first time--there was a 1-inch difference!)
There is a bit at the bottom of the bodice pieces where the front and back are sewn on to each other. I sew the gusset on first to the front or back, then the other side, breaking the stitching at the point of the gusset.
And serge or finish the edges.