If you want a particularly neat finish at the front of the sleigh, trim 1/8 of an inch from the front interior section of the doubled card frame, and fold the longer uncut edge over the cut edge and glue it so that it makes a neat join on the interior side of the frame. This step is not necessary but does finish the front edge. The photos show the sleigh without this finished edge.
Line the front edge of the sleigh frame up with the curve at the front of the sleigh and test fit the frame to the red edge on the interior of one of the side pieces. The frame piece should be end evenly at the front and back. If it does not, adjust where you will start the sleigh front.
Turn all tabs to the inside of the sleigh, and glue the side which will lie against the side of the sleigh. Fit the frame carefully in place, following the curving red line on the front of the sleigh as shown in the photo.Set this aside to dry.