On these pants I actually took in both the hem and sides to turn them from too-long-bootcut to just-right-skinnies, but I'm just going to talk about the hem in this post.
Cut off the original hem giving yourself a generous 1/4" or 1/2" from the stitching. Using a seam ripper, take out the outside pant seam so that your hem piece is one long piece (no longer a tube).
Next, cut your pants to the length you'd like them. Your final leg-length will be where you do your stitching plus the length of the original hem (a half inch or so), keep this in mind.
Using a seam ripper again, take out a couple inches of the outside pant seam (on the right side of the photo above).
Pin the hem onto the pant leg right sides together (your pants should be right-side out) and raw edges on the same side. Match your pant's and hem's outside and inside seams. We will be stitching just below the stitching on the original hem so make sure that when you place it onto the pant leg that it is at this new stitch postion, plus the length of the original hem, that you want your new pant-length to be. Hopefully the picture makes clearer what my words fail to explain.
With your sewing machine, stitch as close to the fold of the inside seam as shown in the photo above.
At this point, if you want, you can finish the raw edges by trimming them and serging or zig-zag stitching. Not doing so won't really change the success of the alteration.
When you've finished this, turn the pants inside-out and stitch back over the side-seam you opened earlier.