Thread your needle with matching, lightweight thread, and sew straight across your hem roughly 1/4 inch (6 mm) away from the raw edge.  After sewing this line, trim the edge so that there is only 1/8 inch (3 mm) between the thread line and the raw edge. This stitch will end up being on the bottom of your hem. It should help you maintain an even, consistent roll. Fold the raw edge toward the wrong side of the fabric and press in place using an iron.  While not entirely necessary, pressing the fold in place will make the hem less likely to unroll as you stitch. Fold the fabric so that the bend falls just past your initial row of stitching. You should see your initial stitching along the underside of the fabric but not from the front. Pick up one thread from the fabric and a small stitch from the edge of your fold. Pull the needle through, but do not pull it taut just yet.  Use a small, sharp needle for best results. Doing so will make it easier to pick up single threads along your hem. The stitch made into your fold should be as close to the actual fold as possible. Position it in between your initial line of stitching and the fold itself. The threads you pick up from the front of the actual fabric should be directly above the stitch made into your fold. These threads should also be right above the raw edge. Make sure that you only pick up one or two threads from the fabric. Picking up more will cause the hem to be more visible from the front of the fabric. Each stitch should only pick up one or two threads from the fabric, and the stitches should be spaced roughly 1/4 inch (6 mm) apart. Repeat this until you have 1 to 2 inches (2.5 to 5 cm) of hem stitching. Lightly pull on the thread in the direction of your stitching. The raw edge should roll itself down into your hem, disappearing from sight.  Use firm pressure, but do not pull tightly. Pulling too tightly can cause the fabric to bunch up. Smooth out any bubbles or bumps with your fingers. Continue stitching along the rest of the hem in the same manner until you reach the end. Knot off the end and cut off any excess thread.  As you get better at the process, you can pull the thread after 4 to 5 inches (10 to 13 cm) of stitching rather than every 1 to 2 inches (2.5 to 5 cm). If completed correctly, the raw edge should be hidden along the wrong side of the fabric and the hem stitching should be barely visible from the front. The hem may already be smooth enough, but if desired, use an iron to press it further. This step completes the process.

Summary:
Sew straight across the raw edge. Fold in the raw edge. Pick up a few threads with your sewing needle. Make a few more stitches in the same manner. Pull the thread through. Repeat along the length of the hem. Iron press when finished.