The first thing is to measure the length of the cuff.
Put the pant on your girl and measure how much more length you need to add to the bottom of the pant leg.
If for example, you need 1,5 inches you add 1 more inch and then double it.
So it would be 1,5 + 1 = 2,5 x 2 = 5 inches.
Then measure the width of the pant leg at the hem level.
Double that measurement and add ½ inch seam allowance on each end.
With wrong sides together, fold each cuff along its width, bringing long edges together. Press, then open up again.
For the bottom fabric, Turn the long sides under ½ inch and then iron it.
Fold each of the cuff pieces in half along the length, bringing short edges and right sides together. Sew along the short edges with a 1 cm (1/2″) seam allowance to form 2 tubes. Press seams open.
Iron the seam allowance open like this.
Fold again putting together the hems like in the image below.
Now find the seam between the legs of your pants and match it with the extension seam, place the hem of the pants between both layers of the new fabric extension.
Pin in place.
The pant’s hem will be sandwiched between both sides of the fabric.
Turning the pants to the wrong side, fold the pants cuff up along the pressed fold. Pin in place, ensuring that the folded edge of the cuff extends about 0.5 cm (1/4″) past the seam joining the cuff to the pants. Then iron it.
From the right side of the pants, ‘stitch in the ditch’ of the join between the cuff and the pants, making sure that you catch the inner edge of the cuff in the underside of your stitches.
Fold the extension so it would hide the seam.