Ok. whew. So, now you have your pants pinned to the right place. Take them off (yes.. yes. dont try to hem pants while wearing them. i've tried. it's a dumb thing to do.) Now back to my original questions. Do you have a serger? If yes, then if you had to cut off some of the length, make sure to leave probably 2 inches to hem up. You dont want a small hem in dress pants, or they will lay funny. If you had to cut some off, make sure you cut equal amounts off each leg (unless the person has incredibly different length legs), and serge the raw edges of the fabric. If you said no to the first question, you will have an extra step. You will need to have hem tape (It is NOT sticky. I dont condone any adhesives in sewing. It's basically just really thin ribbon. The package will say "hem facing tape" or something like that. Make sure it does not say anything about "iron on" or "adhesive" yuck!) and preferably pinking shears (the ones that cut a zig-zag). First use the pinking shears to "pink" the raw edges. this isn't quite as good as a serger, but it dramatically reduces the fraying of the fabric. If you dont have pinking shears, its ok... just skip that step. next you will sew the hem tape to the raw edge of the fabric.
Ok! So i think that's it. To my sewing peeps, let me know if i forgot anything. To my beginner peeps, let me know if i need to clarify anything.