The neck is wide enough not to need a slit. The side width is increased by underarm gussets which meet the skirt gores. The sleeves have a batwing effect which was popular in the 13th century and the skirt is very wide.
1 - Hem the neck hole.
2 - Sew up the straight edges of the front and back gores and then sew them into the dress.
3 - Stitch together the straight edges of the side gores.
4 - Sew the gussets to the top end of the side gores.
5 - Stitch the gussets to the sleeves and stitch the sleeve seam to make up the "side assemblies".
6 - Stitch the completed side assemblies to the sides of the body.
7 - Hem the sleeves and bottom of the skirt straightening up and shortening as needed.