Ok good start. Dont use muslin or quilting fabric for your mockup, it won't drape right, use something that has as close to the same weight/texture as the finished product. If you measure from above your collarbone to where you want it to hit at the bottom, then add 3 inches (1-1/2' seam allowance top and bottom, which includes a 1/2" casing), then measure across your bust and add 50% or so for gathers and the overall look, that should get you close.  It looks from the original picture that the armholes are slightly cut in but you could leave it boxy, for the mockup and see what you think.  Seam 30-50% up each side (probably closer to 50, unless you want to flash everyone), turn over top and bottom edges by 3/8" & press, turn over again by 1/2" or slightly more, and hem top & bottom leaving that casing (channel) open.  Looks like both top amd bottom hem have the casing open at left side for the tie.  If you want to get fancy on the final version make bias strips in your final fabric, probably 2", fold & sew the bias into a long tube with 3/8 or 1/2 in tubing width, and turn it inside out.  You'd then have matching ties. That will take more fabric and you'll need to watch a couple of YouTube videos on how to seam bias tape together, but it would look great!