Use ballpoint needles, this will fix a multitude of problems. Use size 11 for lightweight fabrics, size 14 for medium weightfabrics and size 16 for heavy fabrics
Lighten pressure on the presser foot so the top and bottom fabric feed together
Use a longer stitch length, 8 to 10 stitches per inch, longer for topstitchinu
If you are using a regular sewing machine, stretch the fabric in front and back of the presser foot as you sew5.
Sergers are great for knits, but not necessary. Knits do not ravel, so the seams do not even need to be finished. You can pink the edges or zigzag if you wish
Use twill tape or seam binding to stabilize shoulder seams. (Sew tape into the shoulder seam.)
I usually prewash knits, but sometimes I construct the garment first and make sure I have plenty of length, then I wash and dry it and finally, hem. Knits usually don’t shrink much in width but they can shrink quite a lot in length
To hem knits use a deep hem–1 1/2″ – 2″ to help the garment hang better. Lengthen the stitch legnth to a short baste. Topstitch, holding fabric taught in front and back of presser foot. Stitch again 3/8″ from first stitching