I am not yet over my love of skinnies... Sorry bf jean fashion bloggers....
So I alter a straight or bootcut into skinny.
Take a line from the hip running straight down to about 2 inch above the knee and then slowly tapering to the desired ankle width. (See arrows)
Then try on BEFORE you cut anything. And I try on one leg before I do the second just to check my line.
This is streatch denim but I used a regular sewing machine on a straight stitch. Make the stitch quite short to cope with the pull on the seams in skinny jeans.
All done! Easy