For this I lay the jeans out flat and ,using a ruler, draw a line across where I want to cut. The line should be parallel to the existing hemline and perpendicular to the grain. After the first cut I use the leftover leg and lay it on top of the other to make sure, that the legs have the same length. You will need very sharp scissors.
There are many ways to finish these. Here are two:
Just add two rows of stitching about 3/4 inch above the new hemline. I try to find a similar thread as the original and adjust the stitch length accordingly.
This is the fastest method, but you have to deal with way wards strands where the pants fray for a while, every time you wash them.
Here I added a strip of cotton fabric about 2 inches wide and left it open at the sides. I also finished it with a double row of top-stitching.
Other ways are to turn the hems and top-stitch or you could use bias binding to finish the edge. Or you could add a flounce like in these pants