A woman's pants need to fit just right on her body and should be tailored correctly in order to fit comfortably. Alterations are necessary where specific areas don't fit properly. Maybe the pants are too wide or just need slight height adjustments. All of these changes aren't too difficult to do--just as long as you know the specific areas that need tailoring and exactly how to alter them without ruining the pants.
Model the pants that need tailoring. It is best to have the woman who owns the pants wear them in front of you while you mark the alternations. Use silver sewing pins to adjust the pants in the specific areas that need altering. Most likely, the pants will have too much fabric and will create puckering at the waist or sides of the legs. Fold over this extra fabric along the seam lines as neatly as possible, and pin the area down. Be careful not to poke the woman who is modeling her pants.
Turn the pants inside out. Re-pin the specific areas that needed altering. Do this by pinching the pinned areas on the outside, and then switching over the pins on the inside of the pants. Pin along the lines of seams--not perpendicularly. Have these pins be perfectly form-fitted to the woman's body shape. This must be an accurate representation of the altered area.
Iron the pants according to the pinned areas. Set out an iron and ironing board. Turn on the iron to the proper setting for the material of the pants. Cotton will need a high heat setting while silk will need a low heat setting. Lay the ladies' pants flat on the ironing board. Make sure the pinned areas are flattened at the edges to re-create how the new seams will shape the pants. Begin ironing at the waist and slowly iron along the legs and down to the bottom of the pants.
Take out the seams of the ironed pants in the places that need altering. Use a seam ripper to pull out the thread at the areas where alterations are necessary. You will most likely need to take out all the pins and re-correct the positioning of them. Threads may get stuck inside them or movement of the pants will make them fall out. Pluck out the thread carefully. Brush over the area so that this thread is completely off the pants.
Prepare your sewing machine. Thread the machine with thread that matches the original color of thread in the pants. Position the pants onto the sewing machine. Lower the needle in the place where the regular seams can still be used.
Make a tiny reverse-stitch before sewing at the area that needs altering. Once that's done, begin sewing along the thread of the original seam. This should be exactly on the original thread until you're beginning to move towards the place that needs altering. Start guiding the sewing needle to follow the ironed edge of the pants. You will have to make sure that you're sewing perfectly steady and according to guiding pins.
Sew along the ironed edge and exactly where the pins are. As you sew, you must remove the pins, one by one. Note: this line will curve, especially around hips. Sew very slowly and with a steady arm for straight seams. Sew all the way until you meet the original seam of the pants again. Have the seams intersect as naturally as possible.
Reverse stitch after you intersect with these lines. All the pins should be out. Remove the pants from the sewing machine and cut the thread close to the pants. Repeat this sewing, making a double seam, for optimal durability.
Use sharp shears to cut away the excess material about 1/4 inch from the edge of the new seams. Turn the pants right-side out. Iron the pants again to flatten the seams, just as you did before.