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. For best results, use a needle specifically made for jean material. I used regular thread.
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!