To save yourself the work of sewing sleeves in, it’s easier to knit sleeves right onto sweater and knit them from the shoulder down towards wrist. This is how it’s done:

Depending on circumference of sleeve, you may be able to start sleeve with 16” circular needle, then switch to double-pointed needles as sleeve narrows. Or if sleeve circumference is less than 16” (on a child’s sweater, for example), work entire sleeve using double-pointed needles. (See > How to knit in the round).

Right side of sweater or cardigan facing you, start at the under-arm position and pick up 1 stitch per 2 rows on sweater. If you knit the first and last stitch at armhole edge when you are knitting sweater, it will create a firmer selvedge to afterwards pick up stitches from and the little knot it creates every 2 rows is easy to see. When picking up stitches, put needle through that little knot and pull yarn through from back to front to create stitch (if instructions call for picking up additional stitches; for example: every fifth stitch pick up 1 extra, pick up loop between knots and pull yarn through from back to front to create additional stitch). Pick up number of stitches according to pattern. Mark beginning of round with piece of different colored yarn. Then start knitting sleeve according to pattern. For decreases in sleeves, knit first 2 stitches in round together, then knit in pattern to last two stitches of round, and knit those last 2 stitches together. Always work decreases symmetrically: at beginning and end of same round.

For neckline, use 16” circular needle on adult sweater, double-pointed needles on child sweater. Pick up stitches as follows: right side of sweater or cardigan facing you, on a cardigan start at right front and pick up around neckline and on sweater begin picking up stitches at left shoulder. Pick up 1 stitch per stitch on sweater: put needle through last stitch right before neckline decreases on sweater and pull yarn through from back to front to create stitch. Mark beginning of round with piece of different color yarn.

If you want neckline to lay flat, a good rule of thumb is to knit 2 stitches together in first round at each shoulder (just in first round, don’t repeat in subsequent rounds). Then work rounds as written in pattern.