This will make sense to you if you make the Just One More Row Heartbeat top. I am down to the edgings and fiddley bits.
The sleeve gusset was worked by marking the sleeves out. When I knit the long edge, I knit side, SSK into the first sleeve stitch, knit sleeve, K2tog using the last sleeve stitch and first side stitch. On the return, P2 tog using one side and one sleeve stitch. Then I worked the SSK/K2tog row once more. This sounds harder than it was, and will make sense if you work the pattern.