How do you line a knitted sweater?
Anchor the lining piece to the sweater with pins at the under the sleeves, at the bottom where the side seams are and at the sleeve cuffs where the seams are. Line up the (already folded under) cuff of the lining with the cuff of the sweater and hand stitch into place.
Is lining necessary?
With fabrics that don’t fray too badly, a lining provides enough protection against seams chafing against the body that finishing by serging, felling or French seaming is unnecessary. … Interlining materials tend to be uncomfortable and unsightly, so a lining is needed to sandwich the whole thing together neatly.