I have been altering pants for a customer. I don't know if I do it "by the book." I taught myself how to by taking things apart paying attention to how they were made. I thought some one else might be able to use the technique I use if they have never been taught.
The first thing I do is to have the person try on the pants and then pin them with safety pins holding the fabric where it needs taking in. I put as many safety pins in as needed to check the fit on the rear and front and the hang of the legs. I saw a tailor and a home dec designer both pin on the outside of things on tv shows, that's what gave me the idea.
Next, I turn the garmet inside out and mark the fabric where the safety pin is. I mark both sides of the fabric. Then I remove the safety pins. By the way, the reason I use safety pins is that you don't have to worry about the person getting stuck when taking the garmet off. You also don't have to worry about the pins falling out and not knowing where you pinned after the person has left. The next step is to take apart any waistband and the hem. The two pairs of pants I have been altering have drawstring waistbands with elastic in the back. I took a seam ripper and took apart the waistband a few inches on both sides of the seam. I then took the elastic and pulled it out of the seam. I used a safety pin and attached it to the pull string to keep it from sliding in the casing and getting lost. I took the seam ripper and took out the hem. I did this on both legs.
Once I have marked the position the safety pins were in with chalk and removed any thing blocking me making a straight seam up the side I pin the two layers together with straight pins and take a ruler and mark my sewing line down the length of the pants. Then I sew from top to bottom, with this pair remembering to hold the pull string out of the way so it will not get caught in the seam. After I check the seam I take the pants to the serger and serge 1/4 inch away from the straight sewn seam. This cuts off the excess that needs to be removed and finishes the edge for a more professional look. If you don't have a serger you could either pink the edge or cut off the excess to within 1/2 inch of the seam and then do a 1/4 inch zigzag.
I like to tuck my serger threads under when I put the waistband and hem back in. On this pair I tucked in under and sewed over it when I re-attached the elastic to the side seam in the waistband. Next I sewed the waistband back down where I had taken the seaming out to sew the side up. Last I put the hem back in the pants.
That gives you a pair of altered pants that look like they came from the store that way. On this pair I took one inch out at the waist on each side and two inches starting at the hips down the legs on each side.
Sew Long for Now!