Short answer for shocktrooper and other Davros fans, followed by long answer that anyone can use for any figure (but especially the Star Trek movie uniform):
hocktrooper: A Davros headsculpt and chair have been on my 1:6 Want List for ages. Once I get those, I'm ready to sew. Terry Malloy told me the tunic he wore might have looked like leather on-screen, but it actually was made of satin or a similar material. My best idea for the outfit is to modify the naval uniform pattern from the book "Men's 17th & 18th Century Costume." (There's a more info on that book at my website.) It sounds weird, but read the long answer for why it will work.
Long answer:
Looking at actual sewing patterns, you can see that certain shapes repeat. Get familiar enough with some of the basic shapes, and you can recognize them when they appear in outfits. Then you can mix and match those shapes to create or re-create whatever you need. Most costume designers for our favorite films and TV shows recycle and modify elements of old historical patterns instead of creating something completely from scratch. Why re-invent the wheel, when you can tweak it just a little bit instead?
For example, the black tunic Davros wears has an off-center fastening, as seen in these pictures:
https://www.solentnews.biz/images/davros3.jpg
https://www.behindthesofa.org.uk/files/JE-Davros-Hat2.large.jpg
Hmm, that looks a lot like the Star Trek uniforms in movies like "The Wrath of Khan," as seen in this picture:
https://images.entertainmentearth.com/%5CAUTOIMAGES%5CDC17680lg.jpg
Unfasten either Davros's tunic or the Trek uniform part way and you'd get a flap, sort of like this:
https://www.starstore.com/acatalog/stwok-spock-kirk.jpg
Hey, I recognize that shape! Both the tunic and the uniform look like variations of the old 18th century naval uniform. I made this custom outfit ages ago:
Shorten the coat tails and add some length to the front to create a tunic. Leave off the cuffs. Those basic modifications get you a good resemblance for either a Davros tunic or a Trek uniform. Or for many scientist or Frankenstein style lab coats. Or for sci-fi police or military uniforms.