First start with a pair of jeans that fit you well but are baggy around the ankle. You will also need a pair of skinny jeans that you like.
Turn both pairs inside out.
Lay the skinny jeans on top of the baggy jeans.
Draw a line with chalk using the skinny jeans that fit you well as a guide. The chalk line will run into the seam of the baggy pants near the knee or thigh area. *Note, on my jeans, the new seam is the outer seam of the pant leg.
Place pins between both layers of the baggy jeans so the fabric stays in place while sewing.
Sew with a straight stitch over the chalk line, removing the pins along the way.
Try your new jeans on to see how they fit! Before cutting make sure you like them, stitches can always be undone ;) *Note, some jeans have more stretch than others so this is an important step.
Now you can cut the excess!
Now they are skinny jeans!