Last year, my mother-in-law gave me several yards of linen fabric. One side had a rather ghastly print, but the other side was natural linen color. I made myself three pairs of pants from it, hand sewing in the style of Alabama Chanin. This goes much faster than one would expect, and is very portable.
Today, I used the leftover fabric, about half a yard, to make baby pants. I measured the baby's circumference and length at various points: waist, hip, thigh, knee, and ankle; and also the rise (waist to middle crotch). Then I drew up a quick pattern based on those measurements, starting at the waist and working down and out. I added extra for the waistband, the seams, and the hem at the ankle. Then I added some more for ease (room to move): one inch of circumference in the waist and up to two inches in the thigh and ankle, because the fabric has very little stretch. I made the front and back sides of the pattern identical; little children have little butts. I had enough fabric to cut out four pairs of baby pants. I also had elastic salvaged from old kids' clothes for the waistbands.
The pattern drafting and the cutting out are my favorite parts of the process; the rest is just work. I sewed them all up on the sewing machine. Finishing the hems and waistband are the slow parts of the process. I haven't tried the pants on the baby yet, but I suspect I should have made the rise larger.
This page has a nice primer on the basics of sewing pants. In particular, notice the pattern shapes for the front and back pieces of a pair of pants. I can take my own measurements, draw up the pattern shapes scaled to those measurements, and sew a pair of pants that fits better than anything than I will ever find in a store.
I remember, though, that Susie Bright wrote in one of the former CRAFT: magazine issues, that the best way to get a perfect-fitting pair of pants is to sew yourself a figure-flattering A-line skirt instead. Pants are very tricky, if you're trying for a perfect fit. But basic roomy pants are easy.