Turn both pants inside out and lay the pant to be altered underneath the "pattern" pant, matching hip and crotch seams. Using a pencil, lightly mark the outline of the top pant on the bottom, keeping in mind that you need to pull out the seam allowance and trace around that, as well as add extra space at the bottom to allow for a hem. Only do this to one leg right now. Instead of matching both legs to the "pattern" pant and risking having 2 mismatched legs, we are going to tailor 1 leg and match the other leg to it.
Start above the point at which your pants taper in and run it through your serger, keeping in line with the current seam. Slowly ease into your pencil line until you are cutting away and sewing in where you marked.
Try on your pants. Are they how you want them? Did you ease into your pencil line or do you have a "pointy" hip seam? Adjust any mistakes until this leg is perfect. Don't worry about the hem just yet. I actually had to take the pants off and taper them in more.
Now, let's make the other leg just like this one. You no longer need your "pattern" pants, so put them aside. Fold your tailoring pants in half, matching the crotch seam and inseams. Make sure to flatten out your pants perfectly so that there is no bulk and your two pant legs are perfectly and evenly folded. Trace around the outside edge as you did the first leg.
Repeat the serge-easing cutting process, just as you did to the first leg.
Again, try them on. Make sure both pant legs are identical. If they aren't, refold them in half and redraw your pencil line, serging again. Once your pants are exactly the way you want them, you just have to hem the bottom. I hemmed my pants like denim jeans, so I did a small 1/4 inch hem.
Voila! Skinny jeans from trouser jeans. Refashioning clothes makes me feel like I got them for free. Hooray for new life to old clothes!