As you work down the sleeve you will gradually decrease the underarm gussets, taking care to decrease at the same rate as you increased earlier (usually 2 stitches every 4 or 5 rows).
Eventually you will be left with just a single knit stitch flanked by 2 purl stitches. The aim is to be left with just a single purl stitch, to serve as a fake seam stitch which will run all the way down the sleeve, just like it did on the body.
To do this, when you reach your next decrease point, with a purl decrease knit together the first purl stitch and the central knit stitch; then slip this new purl stitch back onto your left-hand needle and then knit it together with the other purl stitch in another purl decrease. The 3 stitches (two purls and a knit) are now a single purl stitch, which is your fake seam stitch.
Although this may not be the simplest way of decreasing 3 into 1, I find it works well because it doesn’t interfere with the stitches of the rest of the sleeve, which make a clean border all along the edge of the gusset and on alongside the seam.