Okay measurements you will need:
Width of shoulders
Length from shoulder to where the dress ends
Width of neck
Get a piece of paper a bit longer than the length, and a bit wider than half your shoulder measurement.
Half the measurement of your shoulders. Mark where this is from the edge of the paper. (red squares) (the dark blue line is the paper)
Make another mark at the edge of the paper for the length. Measure half of the sweep perpendicular to this and make a mark. Now link up the points. You should have a wonky looking rectangle with the left side of the piece of paper forming once side. (green squares)
At the top, mark the width of your neck plus 3 cm. (from the left side) Draw a curve down to create the cutout bit at the front. (orange and yellow squares)
check all the measurements, then add 1.5cm seam allowance all around, with 6cm at the bottom. (blue line)
Cut out two pieces on the fold. Sew along the sides, then hem the armholes, neckline and hem.
your done! now go enjoy your new dress!!