Steady as She Goes
Keep a steady hand as you make changes to clothing patterns, or to clothing that has been cut from a pattern. Follow the old carpenter's maxim: "Measure twice, cut once." By carefully measuring before you cut -- and by keeping steady after you've taken the scissors in hand -- you'll save both time and material.
The Easiest Path is Not Always Best
If you're taking in the legs of your jeans or other pants, the easiest thing to do is to simply trim fabric from the inseam. However, the easiest thing is not always the best, and if you taper the legs this way, you're likely to end up with more fabric than you need below the rear end and underneath the belly. Trim out some of the fabric from the side and inseams, but don't stop there. Trim the line that defines the crotch and rear end.
Yes, Fashion Matters
Take fashion trends into account before you cut. Baggy jeans, sans taper, may be the order of the day if you are young and male. Show up wearing baggy jeans at a fashionable New York club, on the other hand, and you're not likely to get past the front door. Look at what your friends are wearing and check out the magazines, and you may just decide to take your straight-legged pants and introduce a fashionable taper.
Stages
Treat the thighs and the sections of your pants legs below the knees as separate projects. If you are somewhat apple-shaped, you may find that although the waist fits perfectly well, the thighs are baggy. Mark a gently sloping angle along either the inseams or the exterior seams -- or maybe both -- increasingly narrowing the sag of the pant leg around the thighs by an inch or two. Start another taper at or slightly above the knees, removing fabric along another gently sloping angle, narrowing the circumference of the pant leg all the way down to the ankle.