Originally Posted by MTS

Hmmm, isn't that sort of like trying to fit the toothpaste back in the tube?

How about this.

Batting on the bottom
Next - the top sheet, right side UP.
Then - the fitted (backing) sheet, right side DOWN

Make sure it's all nice and smooth and flat.
A pin or two won't hurt.

Then sew around the outside - either marking the line or winging it (You're going to have to that anyway even it's just the two sheets).

This way you'll be sure that all layers are the correct size.

Leave an ~18" open on one side so you can easily turn/birth it.

Then trim the seam allowance and turn inside out through the opening.
Sew the opening closed - by hand or machine.

Smooth it all out and then further quilt it or tie how you want.