Your pattern will usually set out the order of seaming: normally the shoulder seams are joined first if you have to pick up stitches to make the neck band. There is a choice of two methods, the edge-to-edge seam and the back stitch seam.
An edge-to-edge seam is useful on lightweight knits because it is almost invisible and forms no ridge.
A back stitch seam is stronger and firmer and is suitable for all garments but it forms a ridge.
Place the pieces to be joined edge-to-edge with the "heads" of the knit stitches locking together. Match the pattern pieces carefully row for row and stitch for stitch. Using the main yarn, sew into the head of each stitch alternately.
Back Stitch Seam
Place the pieces to be joined together with their right sides facing. Carefully match pattern to pattern, row to row and stitch to stitch. Sew along the seam using backstitch sewing into the centre of each stitch to correspond with the stitch on the opposite piece. Sew 6mm in from the edge of the knitting.