As soon as all trouser legs have been prepared with pockets, yokes, etc., all raw edges (see yellow line in the photo) can be trimmed. Make sure that the top steps of both front trousers are of different lengths. Now the trouser legs can be sewn together before proceeding with the zipper processing.
Take the front and back pant leg and lay them on top of each other, right sides together. Close the side seams of the front and back trousers with a 1.5 cm seam allowance. Work both legs and then press open the seam allowances.
The inner leg seams are sewn together with 1.5 cm. Don't be surprised if the distance between the buttocks and the knees is slightly shorter on the back pant than on the front pant. Stretch this line a bit so it fits. This cutting trick ensures a better fit of the pants. The trimmed seam allowances are ironed apart.
Next you can close the crotch seam. To do this, turn one trouser leg right side out, tuck this trouser leg into the other leg that is not turned, so the crotch seam lies nicely straight on top of each other and you can sew them together up to 1cm above the lower edge of the top step. Make sure that the yoke is sewn together at the center back at exactly the same height, and that the inner leg seams are exactly on top of each other. The finished seam allowances are ironed about 10-15 cm apart at the center back.
The next step is the slip of the underlay, which you place wrong on the wrong side and neaten it together at the long and lower edge.
Now take your zip and slip to hand. Sew the opened zipper onto the underlay. Do not topstitch too close to the ripper bar so that the slider does not get caught later.
Iron the underlap according to Knips and sew the zip with the facing under the left side of the pants. Use a ripper foot for easier processing.
Iron the flap over according to Knips, then lay the pants neatly in front of you so that the flap covers the zip and pin the flap to the zip with a needle. Open the zipper and then fold the underlay sewn onto the zipper to the side. Sew the zip through the ironed top flap at 2.5cm and 3cm next to the break in the center front.
As soon as the zipper has been attached on both sides, the typical front trouser topstitching and simultaneous attachment of the ironed overstep can begin. Make it easier for yourself by using a quilting template. Start at the top edge and open and backstitch the topstitching 10-11cm, then stop and bartack there.
Now the underlay can be folded back under the closed zip and, depending on the design, in a curve or at an angle to the center front. It is stitched through to the corner of the crotch seam through the top flap, zipper and under flap and locked at the crotch seam.
Have lots of fun with it