I haven't worn boot-cut jeans for quite a time, but two pairs were still lying in my closet. I made the first pair into something between skinny and straight cut jeans. Very easy, just sew along the side seam to narrow the trouser leg. The second pair became shorts.
1. Mark desired length and add 4 cm seam allowance. Cut.
2. Fold twice.
4. Put the trousers on and pin the lace to the edge.
5. Unpin the ends from the trousers and pin them together.
6. Stitch them, right sides together.
7. Fold lace seam allowances to one side and pin.
8. While stretching the jeans slightly, stitch the lace on both lower and upper edge with a zig-zag stitch.