Because there's been so much research into the costume --- the lines of the outift, the trim, buttons, etc., --- I can just concentrate on making the outfit without worrying too much that I'm missing "something". However, I still have to do my own legwork in finding the appropriate color wool for her outfit, a suitable left-facing cameo, and other accessories.
But, first, I have to figure out what pattern I can modify to be close to the Missy costume, which is an interpretation of a late Victorian/early Edwardian clothing. Luckily for me, I have fair number of historical patterns.
So, I went to my historical pattern bin and looked through many of them to see what would work for Missy. I came up with three possible candidates for her jacket.
All three would require modifications, but they are a good basis to start and do the modifications as needed. For now, I might start with the Truly Victorian pattern and make alterations. Why? Because the Truly Victorian patterns are really good patterns, whereas Rocking Horse & Reconstructing History patterns are often quite difficult to decipher their instructions.
For the skirt, I have a Victorian gore skirt from Laughing Moon that should work well enough with modifications for Missy's outfit.
Now, it's just a matter of creating the mockups and getting it to fit correctly and look close enough to the screen version. I still have to look for the appropriate wool for the jacket and skirt, but the mockup should fit well first. I have several months for Gallifrey to finish the outfit. It shouldn't be THAT hard? :-)