Now obviously this sketch is not to any particular scale, and (if your pattern does not have these gussets as drafted pieces) you might want to do some experimental mockups to determine the best width to allow your arm to move freely. This will not affect the armsceye seam or your sleeve, just the back of the jacket. If you have a back yoke, the gusset will need to rise as far as the shoulder seam.
There are two layers to the gusset, which is stitched together to form a kind of accordion pleat between the back and the sleeve. In leather, to make sewing the shoulder and side seam less difficult and lumpy, the under-layer of the gusset (which attaches to the sleeve) is cut wider than the upper-layer (which attaches to the back) and is offset by at least the width of the seam allowance (so as to not have several layered seams all needing to be stitched into one place). It can be offset more as a design element.
The edge/seam allowance/contour of the under gusset needs to be the same as the original armscye edge, with the other piece drafted to be further in towards the jacket body. I would probably sew the upper gusset to the back, turn and topstitch, then sew the under gusset to the upper gusset. From that point you can simply sew as usual, as the back of the jacket is now the same size as the original pattern.
I also used this concept for a wedding dress made for a friend, who wanted complete range of arm motion combined with a fitted set in sleeve. I did not offset the gusset at all, since the silk fabric was much thinner that riding leathers. By experimentation, we determined that extending the gusset under the arm, and gradually curving it away to nothing in the front armscye, would both leave the front of the bodice entirely undisturbed and allow her to move her arms freely both forward and directly overhead. When her arms were down, the gusset folded up neatly and unobtrusively.
This is a Useful Technique and (as far as I know) is not often seen other than on riding leathers...
~ ~ ~≈:::≈~ ~ ~
The cold weather continues to be wearying. Last night saw me falling asleep while still wearing my wooly fingerless mitts, they work well for bedtime reading and somehow just never made it to the nightstand till the middle of the night. The hen's waterers need swapped out every day, as they freeze solid, and the speckeldy chooks gravitate to whatever corner of the yard seems the brightest. While this is not excessive for those who live where this kind of cold is either usual or would be considered balmy, my preference is for the usually moderate misty moisty wintertime that is mostly common here. Give me a good grey rainy day over a bittercold bluesky one anytime.
~ ~ ~≈:::≈~ ~ ~
For the last few years I've been growing on green onions by planting the remnant ends from bunches bought at the grocery, and that has worked rather well. Apparently the same thing can be done with celery! While reading the latest post from The Cottage Smallholder, she mentioned the possibility, written about here, and with details here. As true celery is notably difficult to grow, this might be an easier way to grow small amounts, obviously would not be practical on a commercial scale, but for the home garden...? When the weather warms, there will be another garden experiment here at Acorn Cottage.