after you cut them with the correct hem length you can take little snips is the bottom and create pleats so that the wider part matches the narrow part when the hem is turned up. or you can "ease" the width into the narrow part. You always want to do careful pressing before you sew because that;s how you get rid of the extra width. It's the same concept used for setting a sleeve or inserting a waist band.

You can clip the seams that join the front to the back. This will give you more room. Just be sure to reinforce them so the whole seam doesn't unravel.

I cut about a 1/2" snip in the center fornt, and let the "V" spread for more room.

Or you can take in the pant leg from the hem up a couple inches, and then taper back to the original seam.

OR -

If your taper is extra wide (remember bell bottoms) you can cut your pants to length plus seam allowance and face with matching bias cut fabric or bias tape in a color to match. For a smaller taper, I found that easing with a run of gathering stitches worked better for me but any of the above will work nicely.

i have these pants that when i wear them i have to fold the waist band over so the crotch won't hang so low how can i take up the pant from the waist band

you simply "taper" the seams as needed. IF the very bottom is too wide, you start up just 2 or 3 inches tapering each side in just a little so that it fits. IF the bottom is too narrow to go up right... you start up about 2 in or so and let the seams out so that it's larger