I had to alter my first ever garment. I made myself a cardigan and it was waaayy to big in the body. Luckyly the body was all stockinette. So like altering most things, i turned it inside out, pinned the new measurements then i used the matress stitch to seam it invisibly. It was bulky on the inside and with knitting i wouldn't trust a sewing machine to make sure the sts don't come undone. I used an overlocker, which is designed to cut excess fabric on the seam and neatly cut it and sew it in place to make sure nothing unravels. If you can't use the matress stitch for your piece you could just sew it by hand or machine then use an overlocker.