Working with leather is not as intimidating as it seems if you have a reliable sewing machine and use the correct size machine needle, preferable size 14. Sewing leather, denim and other heavy duty material will require a higher end sewing machine capable of handling heavy fabrics.
Reshaping a bell bottom leather pants
First turn the pant on the wrong side and lay flat on a cutting board or a flat surface
Determine the width you want, pin to hold in place and with a yardstick draw a line from where the flare begin down to the hem on both size.
(When working with leather there is not much room for error, so it will be best to use the measurements of a pant leg that fit you well. Ripping out stitches from leather can ruin the material)
Stitch on the drawn lines, then cut off the excess material
When pant legs are narrowed they fits longer than a flare or bell shaped trousers. At this point you can leave as is or hem to a desired length.
Hemming leather could be a challenge. The hem can be cut in a straight neat line and worn without hemming or use fabric glue to hold hem in place.