you generally sew all except one seam with the piece turned inside out. then you flip it and stuff it and finish that last seam. you have the edges of both sides folded inside and pinned in place, and when you pull your stitches tight it looks just like all the other seams.
So I get how you can internally sew all seams except the final one - how do you do it?
Ladder stitch!
https://www.easysewingforbeginners.com/project/how-to-do-a-ladder-stitch-or-invisible-stitch/
Pure conjure: stich it, but leave the stitches extremely loose until you turn it right side out.
you generally sew all except one seam with the piece turned inside out. then you flip it and stuff it and finish that last seam. you have the edges of both sides folded inside and pinned in place, and when you pull your stitches tight it looks just like all the other seams.