Sewing a hem onto silk fabric can be a challenge. Silk is a delicate fabric that shows nearly all forms of stitching. Machine-sewing a hem onto silk may cause puckering of the bottom hem, which is why many professional sewers always hand-stitch silk hems. Always choose a thread that matches the fabric as closely as possible. If you can't match the thread exactly, then use a thread that's slightly darker than the fabric.
Fold the bottom edge of the silk up one-eighth of an inch toward the inside of the fabric (the side of the fabric that will not be seen when wearing the garment). Iron the fold into place using an iron set to the “Silk” setting.
Fold the hem up an additional one-eighth of an inch and iron in place. Do not pin the hem, or you may leave holes in the fabric that are hard to hide later. Ironing should be enough to hold the hem in place until you begin sewing.
Thread the needle with a single length of thread only. Place the end of the thread inside the hem fold. Do not make a knot in the end of the fabric; eventually, the stitching will hold in place on its own.
Pull the needle through the hem from the bottom up to the top. Place the needle as close to the first stitch as possible and bring it back through the other side of the hem. Position the next stitch about one-half of an inch away from the first. Push it through the hem to the front side of the fabric, then position the next stitch as close to the most recent stitch as possible.
Stitch two extra small stitches in the fabric every four inches of hem. This strengthens the hem and prevents the entire hem from pulling out if a stitch gets loose. Add new thread by working it under the hem just as you did to start the hem. When you reach the beginning of the hem, stitch two small stitches just as you did at the end of every four-inch interval.