Onto the back, draw the new neckline: raise one side of the collar and draw a line almost parallel to the shoulder for the asymmetrical style.
Draw the waistline and below that draw the skirt panels, reducing a bit at the knee and glaring at the hem for the mermaid skirt.
Onto the front, do the same alteration at the neckline, move the shoulder dart to the side seam, reduce the skirt at the knee and flare it at the hem.
Then, draw the slash lines for the front skirt drape.
Start from a basic sleeve and cut off from the sleeve cap the same measurement lowered on front and back armhole.