lay them out nice and flat. let each of the pieces overlap the other(s) by about 2". Using scissors or a rotary cutter, cut through the overlapped layers using a wavy line. i know it sounds wierd but - trust me - wavy is better. then pull away the scraps. you'll find your pieces are perfectly matched and flat.
you could hand-stitch them together - as though your were stitching up a cut.
you could try forming a temporary bond using freezer paper, then take it to the machine where you'd use one of the mending stitches. just tear the freezer paper away when you're done.
lazy thing that i am, i'd get the lightest weight, single-sided fusible webbing or interface i could find and use that to stick 'em together. i'd either reinforce it on the other side with a really wide zig-zag stitch, or just us more of the interfacing. (that would depend on how much bulk it added to the blanket.) safe to do it to both sides if you're machine quilting.
we all make mistakes here. as for me ... i make 'em here, there, and everywhere! :wink: