The avental is made of a lower, colorful panel attached to an upper, cartridge-pleated panel. The upper panel is made of the same handwoven material as the skirt. The cartridge pleats are set with red embroidery thread; in this photo, the pleating threads are visible. As usual, a zig-zag stitch is used to join pieces, and a straight stitch to join the upper and lower pieces. The upper and lower pieces are first joined, then the upper piece is cartridge pleated. The waistband is bound with red cotton tape, which has been hand-stitched down; the tape forms the waist ties.
You can see information about this and other varieties of regional Portuguese costume at the Folk Costume & Embroidery blog.