Aha! Figured something out.

I attempted yet another pair of pants for The Boy, made out of yet another pair of my old pants, and they were successful eventually, but this time I THINK I actually learned from my mistakes, and possibly next time I will tip the balance so that I am spending more time sewing than I am ripping out seams.

The silly thing that it took me so long to realize is this:  To reuse the side seams when recycling old pants, I have to remember that on one side of the seam is the FRONT of the pants, and on the other side is the BACK of the pants.  This has screwed me up in the cutting EVERY TIME so far, because usually you cut out the front pieces together (which are slightly smaller) and the back pieces together (slightly bigger), but when you're reusing the side seams, if you do it that way you will end up with one big leg and one small leg.  Not the look I was going for.

This time, I figured it out in time (that is, after cutting it out wrong but before sewing it all together), but it meant I had to make the pants narrower all over, so that both legs would match.  This didn't stop me from sewing them together inside-out first, however (hence all the seam ripping).

They came out all right in the end.  They even have pockets!
(once again, from the tutorial found here)

He would not put down that dang juice cup the whole time I was taking pictures.
Doesn't he know that modeling is the price he pays for getting new pants?

So here's my tip:  Cut the original pants in half down the middle (along the crotch seam).

Two separate legs

 Then finish ripping (or cutting) the inseam out so you can open the legs up and lay them flat:

Now you're looking at the inside of the pants.

 Then you can lay the two leg pieces out one on top of the other, and lay out both the front and back pattern pieces at the same time so that you don't mess it up!

The pattern pieces meet up at the existing outseam of the old pants.

 Hey, you can even cut it out right-sides together so that you don't forget to turn it around before sewing them together!  I'm a regular fountain of tips.

Right sides together, pinned from the waist to the crotch.  Don't sew all the way down the legs, like I  did the first time--you'll end up with a very strange tube.

Check out the tutorial I linked to above (and below) to see how I got to this point...oh, the suspense!

This brown pair of pants (the fourth pair I made for The Boy, in case you're keeping track) is the first pair I made without ANY MISTAKES!  I didn't have to rip out any seams except when taking apart my old original pants!  I'm growing, folks.  In case you missed it above, here's the pants tutorial I have been using.

MADE basic pants tutorial

One final tip I figured out--if you want to add pockets to the pants (see the kid pants with pockets tutorial), you can still use the existing side seam--just carefully rip an opening in the side seam with the seam ripper that's big enough for the edge of the pocket, insert pocket, and re-sew using the machine along the same stitch line.  Okay, I'm done now.

I promise I won't put up a new post every time I make The Boy another pair of the same pants.  I try not to brag.