This is the method I have used many times to copy a garment I did not wish to take apart to copy.
PANT FRONT AND BACK:

1. Do your best to determine the lengthwise and crosswise grain of the fabric and mark those lines with pins,chalk, or tape. You need a definite line to measure from. On the paper you are using for a pattern draw those same lines, making sure you have sufficient room on each side to allow for the pants width.

2. Mark the grain lines in inches (this is where tape would come in handy) and mark the lines on the paper to correspond with those markings.

3. Now begin measuring. I measure on each side of my marked line and transfer the measurements to the paper. Depending on the shape of the garment you are copying you can space measurements every few inches except where there are more curve.

4. To check my measurements I do measurements of the garment as to front waist, front hip (at 3", 7", and 9"),
   crotch line, knee, bottom. I do the same set of measurements on the back garment. Also, do a front and back crotch length.

5. Connect the dots on your paper pattern, add seam allowances, hems, any other details from the garment.

6. If there are other parts, such as pockets, do same way as the main part of the pants. Measure your waistband and draw that out on your paper as well.

7. Remember that clothing sometimes has ease and so one measurement may not quite seem right, but take a good look at your garment to be sure that it may not have some fullness at the waist or the back leg up by the crotch.

8. The first pair you make you may want to do as a test garment to make sure your measurements were correct and you have the method of assembly correct.

Good Luck

Thank you, Bevy4
Good sounding instructions. Now how do I measure where there are darts? I'm guessing that I allow extra width, as for a 3/4 inch dart I would allow 1 1/2 inches.