Takes a bit of thought but quite easy if you’ve done some machine sewing before BUT remember only sew one square gusset on the liner so can turn it inside out through the unsewn gusset (corner) then hand stitch the second one inside.
Sew & Quilt-in-One Placemats; FLIP & SEW METHOD (straight strips): Layer backing + batting + strip RS up + strip RS down. Sew seam, then flip strip over RS up. No extra quilting needed if strip is narrow. Continue adding strips and flipping them over.