Flip it inside out.
Lay an existing pair of skinny pants/jeans over top (also inside out).
Line up the inner seams from leg to crotch.
Get yourself some pins and chalk.
Adding a little extra for seam allowance, trace along the line of the skinny jeans. Then pin straight and flat. Do this to both legs.
Sew all along the chalk line. Now try them on, while inside out. Be sure it fits well and there's no bubbling. If so, go back and pin and sew. If all is good, move on to the next step.
Now cut off the excess, leaving about 1/2 inch.
Serge the ends or use a zigzag stitch like I did, so it won't fray.
Put them on and be sure it fits fine.
Yay for new skinny cords!!! Gotta love free new clothes.