Let's begin. It takes very little fabric to make panties--less than a yard, certainly.

Lay your pieces out on the fabric. Fronts and back are cut on the fold. This crotch piece is, as well. I cut my real crotch pieces from old t-shirts--but I forgot to take a picture.

On patterns with a cut-on crotch, turn the top of the crotch lining under and pin in place on the panty front.

If you are using a pattern with a separate crotch piece, place the outer fabric pieces right sides together. You'll have to pin some to get the shapes to match up. The crotch piece has a convex curve, while the front piece will be straight or slighly concave. If you are using a lining, pin that piece with the right side facing the wrong side of the front panty piece. Sew with a 1/4" seam on either your conventional machine or a serger.

Topstitch the crotch lining in place. I'm using a twin needle here, but you can use a zig-zag stitch or whatever appeals to you. This needs to stretch, so a plain straight stitch might not work as well.

If you are using a separate crotch piece that you seamed to the front in the last step, you can skip the topstitching.

After the front crotch is sewn, flip the back and front pieces to the inside and pin the crotch pieces right sides together sandwiching the back piece in the middle. Be sure that the right side of the back piece is pinned to the right side of the outer crotch piece. This is kind of a jelly-roll thing(!) and easier to do than to demonstrate.

After you've stitched the back crotch to the back panty, turn it all right side out. Press it lightly, if you like.