You'll need the pants (jeans are best), couple of little tubes of acrylic paint (color of you choice), textile medium, permanent marker, Ibuprofen and the steady hand. Maybe a toothpick and a paint brush.
Even though you are painting free-hand, it does require keeping some lines straight for a maximum "resemblance" (yes, the rip is painted as well for a maximum effect).
If you are painting out of the tube: you should mix the medium (instructions on the bottle) and the paint (squeeze out half of a tube into an empty one and add the medium into it). Steadily paint by squeezing the mixture onto the fabric.
If you are using the paint brush (more labor intensive), mix everything and use either the brush or a toothpick.
Suggesting you paint the front (or back) first; dry for 72 hours; iron from the inside; only then moving to painting the back (front).
Permanent marker might be useful to tint the lace area, as it smudges a bit but creates the desired lacy effect.
Fun to wear around Halloween (and not only).