Hemming pants can be difficult when you do not have an extra pair of hands to help with marking, however I found the following works well:
Try on the slacks with the shoes you will most likely wear with them the most and see where they fall at the top of the shoe.
Tuck the fabric under for a new hemline at the bottom of the slacks and pin once at the front and at the back on the bottom of each pant leg where you want the new hem to lay. Take them off and look at where the hemline will fall. If you must open the existing seam to achieve the new hemline with about a 1 � to 2 inch hem, then open up the existing hem with a seam-ripper or scissors. If you have so much extra fabric at the bottom of each pant leg beyond the new hem-to-be, then no need to open the existing hem as it will be included in the excess fabric to be cut away.
Lay the slacks neatly flat, perhaps on your ironing board as it will be at a nice height to work on the hems, and align each pant leg one on top of the other, from the inside crotch seam on down to the bottom of each pant leg, matching the inside seam of one leg to the inside seam of the other, as if you would iron the slacks with both legs together to form an identical neat crease line down the front and back of each pant leg. With the pants still laying flat, one pant leg on top of the other, tuck the new desired hem of each pant leg neatly under all around allowing the two pins in each pant leg to form the new desired hemline. Adjust each pant leg at the bottom so that both new hemlines match in length. Add more pins to hold the hems in place.
Now you can either cut off excess fabric beyond the desired 1 � to 2 inch hem depth on each pant leg (keep the pins in place when doing so) or open up the existing hem seam if not enough extra fabric is available. Use what you like to finish off the raw new hemline with lace seam tape or bias seam tape in the same color as the slacks for a nice flat hem that wont ravel and allow the hem to look neat, or serge or zigzag the hem edge if your fabric type allows and you prefer to use just a sewn machine edge to prevent raveling of the new hemline edge. If you choose to use the seam tape, you can place the hem tape flat on top of the hem edge and sew them both together to cover the hem edge, or you can first edge stitch the hem edge (straight stitch or zigzag) and then add the seam tape over the top of the hem�s edge stitching to visually cover the hem edge. Very lightly press the new hemline in place to carefully set the new hemline edge.
For dress slacks, it is best to then hand stitch the hem in place using matching thread and small evenly spaced stitches that take a bite out of the hem and only a few stitches in the garment leg. Do not pull the thread too tight or the hem will looked puckered on the outside. Check your progress frequently on the outside of the pant leg as you carefully hand stitch your way around each pant leg to be sure you are not taking too many stitches into the outside fabric, or two large a hand stitch that will show greatly on the outside of the pant leg at the hemmed line. Catching just a few threads of the outside pant leg will hold the hem securely.
Some carefree cottons, jeans and kids clothes do not need this much attention to hemming or even the suggested 1 � to 2 inch hem. You can just machine top-stitch around the pant leg bottom if you wish (I find it easier to turn the slacks inside out so I can top-stitch evenly from the hemline edge on the outside of the slacks), once the proper hem length has been achieved and the hem edge has been edge-stitched to prevent raveling and fraying.
Fabrics with much stretch do better at hemming if you choose one of your sewing machine�s stretch stitches to stitch around the hem rather than hand stitching the hem to the garment fabric. Just following the same instructions for measuring and determining the hem length (look at other similar finished hems in similar garments to see what hem length looks best), and then machine stitch on the outside or inside of the hem as you wish; no hem edging is usually necessary as many stretchy fabrics do not ravel and fray like woven fabrics tend to do.
If your sewing machine has a blind hem stitch, you might want to use that instead of hand-stitching the hem in place. Consult your machine's manual for how-to instructions using this machine stitch.
You might like this product: An iron-on hem-tape from Wrights
http://www.wrights.com/products/sewing/pkgtapes/608.htm
