The easiest way to make a ring blank (the starting point for carving your ring) is to use a wax ring tube. Matt Wax make ring tubes
in three different wax hardnesses:
Blue - Soft, flexible
Red/Purple - Medium, flexible.
Green - Hard, strong.I used a 1 1/8" diameter flat top purple ring tube.
Referencing your drawing, use the pair of scribers to draw a line around the whole of the tube measuring 1mm wider than the maximum width of your ring
(the 1mm extra is a safety margin for filing and sanding.)
Using the jewlers saw and a fine spiral saw blade cut your rough blank free from the tube
Cut 0.5mm further out from the scribed line, keeping the line on the inside of the blank (this gives you a margin to file off the worst of the saw marks). Don't try and cut straight through the tube all in one go, instead work your way around the tube paying careful attention to the location of the scribed line.
An easy way to cut a truly square ring blank is to use a mitre box and saw
Once you've cut your blank free, use the larger flat file to remove the worst of the saw marks.
Then use the ring sizer to scrape the inside hole to the desired finger size.
Keep the hole about 1mm smaller than the finished size to allow for shaping and sanding.