Why would you want a french seam?
I guess there are many reasons, but for this purpose, its because this item (crib sheet) will probably be washed a hundred times and we wouldnt want any fraying.
A french seam completely hides the raw edges.
So here is what you do.
go ahead and stitch your seam right sides together as you normally do ( I do mine twice). Then topstitch on the outside with your seam pinched together. So you get this puckered piece on the outside. Now the picture is my very first french seam, and the next time I do it, it wont be quite so wide. I will do a very small seam.
I’ve been looking at some of my clothing especially knit sweaters and such and am seeing alot of french seams that I never noticed before.