Start by putting your jeans on inside out and pinning the excess fabric to be the fit you want. This tutorial says to lay a pair of skinny jeans you like on top of the baggy jeans and trace the shape with chalk, but I didn't have any skinny jeans so I winged it.
Are you surprised?
Make sure you're able to take the jeans off and put them back on! Otherwise your feet may not make it through the pant leg after you sew the new seam.
Now draw the chalk line along where the pins are pinned and sew along with whatever color thread you want. You could be a perfectionist and use gold thread to match your jeans', but I just used white because no one's going to see it.
Try the jeans on again after you sew them - it's easier to take stitches out than replace cut off fabric! Once you're sure about the fit, cut off the excess fabric and you, my dear, have a new pair of skinny jeans!
Those are some sweet jeans! (get it? of course you do.)