You will need:
- A pair of thrifted trousers
- Some fabric for the pants if you want them to be a different colour/ fabric
- Some adjustable elastic (also called maternity elastic, the one with the button holes)
- two buttons
First cut the waist, about 1/4" below the actual waist bit.
Cut out your pants, either from a pattern or from a pair of existing pants. For a great tutorial for making flat front pants check out Made here, or for basic pants here.
Sew your pants together. If you have an overlocker, serge around the waist.
Take the waistband and place the front of it to the front of the pants. If you need to, pin them and go all the way around until you get to the back. Pin the two back ends together and cut the ends off allowing for about 5/8". Serge the bottom of both ends, then serge the ends you just cut. Sew the ends right side together.
Pin the waistband around the pants' waist, right sides together, and sew. Make sure the seams around the back are folded back on each side, as they will later be sewn to hold the elastic together. Turn the waistband out and sew around where the waistband joins with the rest of the pants.
Now comes the fun part: the elastic! Pinch the fabric on the inside of the waistband at the back of the pants as to separate both sides. The hole should be hidden by the seam. You want to put your elastic in the casing without making a visible hole outside. Once you've got the inside fabric only, cut a small hole with some scissors. Repeat for the other side.
Next decide where you want the elastic to go. I leave about 2/3 of the pants at the front to be flat, which determines where to put the second cut. Once both ends re cut, you're ready to put the elastic through.
Cut the elastic slightly shorter than the length inside the casing. Insert it through the back and when you get to the end, fold the seam over and sew it all together. Then sew a button that will fit through the buttonhole at the front end, pull your elastic through and attach it to the button. Repeat for the other side.
And you're done!