The main package used for clothing design is Clo. It's a full 3D design program with cloth physics. You get to see full outfits on simulated models. There's also Marvelous Designer, which is Clo minus the part that outputs cutting patterns. That's used to create clothing for animation.
You can adjust the mannequin to your measurements and even load mocap data (which was very cumbersome at the time though) to see how the piece may behave when worn.
I had the pattern cut out from paper on a large format cutter plotter. Then I went to a tailor and told them: make it so -- all the details are up to you but stick to the pattern. Of course the pattern was missing stuff like belt pocket, coin pocket, lining etc.
They were a tad bewildered but the pants fit perfectly and I've been re-using this pattern now for over a decade.
For me it was an amazing experience and I'm still marveling (pun intended) at the fact that a zero-tailoring-knowledge layperson like I could do something like this with such software.