Lay the pieces flat and make sure their measurements agree before reaching for the yarn needle. A seam cannot easily fix a major mismatch, but it can make two compatible edges feel like one fabric.
For vertical stockinette edges, mattress stitch is often a useful choice because it joins the pieces invisibly from the right side. For shoulder seams, a firmer method may give the garment useful structure.
Work a short section, check the front, and adjust your tension before continuing. A seam should lie flat without pulling the fabric into a ridge.
