First - my 'soapbox' - I think for women - whose tops lap right over left - that the lap on the trouser should similarly lap the same direction:  right over left.  That said.....

IMPORTANT:  Always buy a 9'' nylon coil zipper - even if you desire only a 7'' opening..you'll see why''make sure center front is parallel to grain line of front.  If it is not - you need to make it so, and take front tucks deeper or a dart to make up any difference.

1.  Cutting:  change pattern front so that you have a 2'' extension from the center seam and 9'' long.  For accurate and ample top edge when the extension is folded back on itself, fold this extension back on the original cutting line on pattern to trace the top edge.

2.  Mark -  the following points:

•  Point A which is the center front (CF) seam line.  Snip in at top edge to mark.
• Point B - 5/8'' out into the extension from Point A - (which would be the cutting line if the extension had not been added).  Snip in at top edge to mark
• Point C is on the seam line, 3/4'' above intersection of extension and crotch cutting line.  Stick in a pin, and mark that point with a Frixion Marker (will disappear with heat!)
• Interface the extension area on the Right Front as shaded in the diagram.

3.  Stitch the center front crotch seam from point C to within 1'' from cut inseam edge.  Do this as the entire crotch seam should be sewn after the inseams are joined, for wearing comfort).

4.  Press extensions as they will lay - see my first comment:  women's zippers lap right over left...

• Press 1 3/8'' under on the left front, or a line from the clip at Point B down to the raw crotch line edge.
• Press under 2'' on extension on the right front, or a line from the clip at Point A down to Point C.

Next, get situated for easy sewing:

• Attach zipper foot to the left of the needle
• Lay the work with the top edge of the fabric towards you.
• Realize you will ALWAYS stitch from the bottom to the top of the zipper.

5.  Lay the folded underlay on the left front next to the left side of the zipper coil, placing the bottom tab of the coil below marked Point C.  Stitch 1/8'' from the fold.  Now you see why a 9'' zipper is best:  the tab will be laying way up off the edge of the pants at the top (towards you), so there is no bump for you to maneuver around.  Remember, stitch from the bottom of the zipper to the top waist edge.  See Diagram above this step.

6.  Lay the overlap, or right side down over the left side matching up Point A clips.  Pin through all layers on the fold as they will lay when finished.  Fold the right side of the pants over to the left.  With the zipper foot now to the right of the needle, stitch through the zipper tape and the right extension as it lays on top of the zipper.  You will be able to 'feel' the teeth through the extension, and the  stitching will be along the right edge of the zipper coil.

7.  Flip the right side back over to the right, so that the right side is right side up.  Topstitch through all layers 1 1/4'' from the center front fold, beginning with a curve at the bottom Point C.  The bottom zipper stop should be below Point C, causing no problems in stitching.  Leave long threads so they can be pulled to the inside and knotted.  Use a Frixion heat-disappearing pen to sketch in this bottom curve and then a straight lie on which to stitch.   You might even want to create your own 'template' to lay along the folded edge along which to draw a line.

8.  Be sure to have the tab pull on the zipper pulled down (the zipper open_ when the waistband  or facing is applied.  The waistline seam will be sewn OVER the zipper coil, shortening it exactly even, and leaving no unsightly 'h***' between the top of the zipper and the bottom of the waistband.