Knit fabrics are difficult to sew; they can be slippery or stretch while you're sewing, and these factors can make it difficult to have the even seams and original shape that come easily when sewing with other fabrics. Beginning stitchers can easily feel overwhelmed, and even advanced seamstresses can get frustrated very quickly when dealing with knit. Since knit is a popular fabric choice for everything from exercise clothing to children's hoodies, every person who sews will likely have to work with knit fabric at some point.
Use extra pins. One way to keep knit fabric from stretching and sliding while you're sewing it is to use at least twice the number of pins you would normally use on cotton or polyester blend fabric. It also helps to use extra-long pins (like quilting pins); they're less likely to fall out of the stretchy fabric.
Get out the wax paper. If the knit fabric is sliding around too much, try placing a sheet of wax paper between the two layers of fabric as you stitch. The wax paper should help hold it in place and will tear away easily when you've finished the seam.
Sew with a zig-zag stitch. Since knit stretches and "gives," you will achieve better results if the seam stitches also have some stretch and give. You can also use the zig-zag stitch feature on your machine to finish the edges of the fabric so they don't unravel.
Take it slow. The best way to prevent stretching and sliding while you sew is to stitch much slower than usual. This allows you to better control the fabric, and it is less likely to be pulled in a way that will prevent a smooth, even seam.