In response to my plea for help, several of you responded (thanks!!) to reassure me that gussets are not really all that mysterious and that I could do it. I am not all that afraid of the sewing of the gusset - what has required more mental effort is the notion of exactly where the gusset should go, and what shape and size it should be to make this coat actually wearable. This problem brought the sewing on the actual coat to a crashing halt. I backed up to muslin stage. If only I had started with such a step, instead of having to down tools in mid-project to "fix" a bad pattern! (Will I ever trust Burda again????)
I assembled a muslin out of a rather stiff canvas-y fabric so that I could test my gusset-making skills. I've taken photos along the way to document my attempts, failures and "progress". It does not really feel like progress when you start to progress from 3 steps back!
Here are my muslin demo pictures:
The next step was to figure out how far into the jacket body to slash so that I could insert a gusset.
I did this very unscientifically! Wearing the muslin and looking in the mirror, I took a white marking pencil and located the top of the slash by feel/instinct where it seemed that if opened up to that point, the "hinge" connecting the sleeve to jacket body would give me enough room to move. Then I had my husband figure out where the corresponding point would be on my back.
This turned out fine, somewhat to my surprise.
On the left sleeve (right of the photo) I have inserted a gusset which turned out far too big. I did not modify the original underarm seam. With that very low underarm, I needed the gusset to be this size to achieve an acceptable range of motion for my arm. However, it is UGLY!
I look a bit happier with this experiment now.
Wish me luck translating this change, and a few others I thought of while puzzling over this problem, into my actual coat.