All you need besides your standard sewing supplies (sewing machine, pins, black thread), are a pair of black flare jeans (I swear H&M always has these on their sale racks in-store–I snagged some for $9.99, but they aren't online) and about a yard of wide lace trim. I got trim with elastic, since it was only $3.99 a yard compared to $10+/yd, and then used a seam-ripper to remove the elastic band.
Now that you've got all your components, let's get started!
First, cut the bottom flares off of the bottom of the jeans, so the pant length falls about 6 inches above your ankle. Prep for this by rolling up the jeans, while wearing them, and finding a good length (keeping in mind that the bell you sew back on will add length. Be sure to measure up from the bottom of the hem, so you cut off an even measurement all the way around each flare. Also, I feel like I shouldn't have to say this, but lay the jeans flat before cutting them; don't cut them while you're still wearing them. ;)
Next, turn your jeans inside out and put them back on. Pin the inner seams from the knees down to create a slim fit pant. The drama will be in the attached flare we add back on, so taking the flare out of the top half is needed to create contrast.
Sew the inner seams of the pants and cut off excess material. Make sure they fit well right-side-out and set aside.
Now, grab your lace! Measure the lace out, so the length of each piece is the circumference of the bottom of the flare. Pin into a circle and sew together, to create one band of lace for each.
Next, turn your detached flares inside-out, pin the lace around the bottom hem, with about 1/2 inch exposed at the bottom. You can always go in and cut more off the hem later if you'd like. Pin the lace around the hem of the pant and sew into place around the upper edge of the lace using thread that perfectly matches the jeans.
Then, trim the tops of the detached flares so they measure about 6 1/2 in. tall. You can mock these up on the pants first to see where you'd like them to fall and customize the height.
Using a large needle and sewing by hand, double up black thread and make a large stitch around the top perimeter, about 1 inch down from the top of the flare. Pull the thread tight, so the fabric bunches and the inner circumference is the same as the slim leg pant you'll be attaching it to. Knot the thread and cut. Repeat for second flare.
Now, all you have to do is attach the flares to the pants. Pin the flares on where you want them to fall (probably a good idea to try them on again). Use a sewing machine to stitch over the bunching stitch you just made to attach both of the flares to the pant legs.
Last, you have the option of trimming the bottom of the jeans to create an uneven hemline, and expose different amounts of the lace around the perimeter. (I cut mine so the pant is slightly longer in the back than the front, or you could make more severe choppy cuts for an edgier look.
You're done! You've now got some ultra-stylish jeans that check off boxes for multiple Spring trends: flares, cropped pants, ruffles, and lace. No one will know that you've made these trendy trousers on the cheap and that they didn't cost and arm and a leg!