Anyhow, to do this seam, first be sure you have extra fabric. For M, who is a size 2T, I usually use just one piece of fabric for her skirts with one seam. I would need an extra inch to do this seam. If I were using 2 pieces of fabric for the skirt or dress skirt, add 1 inch to each piece.
Place fabric right side OUT and sew seams.
Now trim seam closely to the stitch.
Turn fabric inside out and iron seam/s down. Now sew along the seams with right sides IN or facing.
Turn fabric right side out and pull against seams to check for any raw edges sticking out. If there are, then turn right side in again and sew further away from edge til no raw edges can be seen from the front side.
The above picture is what your inside seam should look like when finished.
Here is what it will look like from the outside.