I have tapered several pairs of pants, with really good results overall. It's really quite easy, even for someone with little to no experience. I learned from this tutorial. It has lots of great photos and explains each step in detail. If all you're going for is a taper from the knee down, you should be able to do that no problem, as issues only really arise when dealing with the crotch. After you figure this out on a pair or two, you'll be blazing through the whole procedure in maybe 15 minutes. Plus, you'll save the $30 dollars (or likely more) that a tailor would charge for the job.

Check out some of his other articles if you're interested. There's another tutorial on slimming shirts, which turns out to be very similar, and similarly effective.