How to Sew a Hem
Hemming prevents the edges of fabric from fraying and creates weight so that fabric drapes properly. The single-fold hem is the simplest: the edge is folded over, pressed, then edge-stitched at the top of the fold. Yet most of the projects in this book rely on a double hem--the edge is folded over twice--to produce a ravelproof edge.
Source: Martha Stewart
With a seam gauge and tailor's chalk or a disappearing-ink fabric pen, mark a dashed line the distance you want the first fold to be from the unfinished edge of the fabric; continue along the entire hem. Use a clear ruler to make sure the hem is even as you make the marks. Repeat this process for making a second dashed line where you want the second fold. To make an even 1/2-inch (13mm) double hem, for example, make the first marks 1/2 inch (13mm) from the unfinished edge. Some projects will call for an uneven double fold; for example, the hem of a curtain may be turned over 1/2 inch (13mm), then turned over again 2 inches (5cm). In this case, your marks would be 1/2 inch (13mm) from the unfinished edge and 2 1/2 inches (6.5m) from the unfinished edge.
Fold the fabric over at the first mark, and press. Then fold the fabric over at the second mark, press, and pin. For an even double hem (see illustration) , the first fold will be exactly the same size as the second. For an uneven double hem, the first fold will be smaller than the second.
Edge-stitch 1/8 inch (3mm) from the top fold, removing the pins as you sew. Use the markings on the machine's throat plate to ensure a straight seam.