the best way to draw clothing on a figure is to draw the figure first unless you are intending on doing a one dimensionsal flat drawing, such as a pattern producer would refer to in creating a pattern. Thus needing to be clearly defined and simple to follow the idea of the clothing.
It sounds like you want to draw clothing on a croquis figure. Practice drawing the human figure first. The best way would be to take a model drawing class or get a friend to model for you. At first, don't attempt to draw the clothes, but just get the position of the croquis figure you are following down and practice doing that in different positions. Such as bending one knee or with arm on the hip or behind the back. And pay close attention to where the weight of the body is being positioned to, because this will not only help you to draw the body accurately but also hugely impacts the way that the clothing will fit on the figure and how it hangs.
I'll try to explain the basic procedure for drawing a croquis figure, not so much in fashion Illustration proportion, but simply as to do with positioning of weight and clothing:
Say you copying something out of a magazine...Take a pencil and mark the very centre of the model's neck then draw a straight line vertically down from there. Put this centre line on the page you are drawing. This shows not only how far apart to space the feet but more importantly which leg the model is putting her weight upon. Because it will be the leg most lined up with the centre line. When a garment hangs upon the model's figure; the folds will fall towards the leg that is holding the weight, therfore causing the hem to sit lower and the hips will usually fall in the opposite direction, not only causing diagonal strecthes in the fabric but also adding a curve/wrinkles at the waist on the side the weight is positioned upon.
After drawing the centre line on your paper and lining it up with the centre of the models neck in the picture u are copying, position the legs proportionally and then on the magazine, take your pencil and draw dots on the models two shoulders, then connect them with a straight line, then on her bust points, and connect those also with a straight line, then do the same on her hips. if you repeat the same on your drawing before you draw it, it will help to show where the material will gather or wrinkle and where the hems hang higher or lower.
I'm not sure if that made sense. Because I can't give you a visual picture. And I've never tried explaining that to someone online. lol. But I was just trying to give you a very basic overview to drawing a figure and the way clothes sit upon the body. It's hard to do that simply in words, obviously.
grr...i wish I could draw a picture for you. As they say, a picture is worth a thousand words

Oh well, I hope I helped in some way and that I somehow made a bit of sense. If not, sorry.
p.s. Some good books I've used and would reccomend are:
*9 heads : a guide to drawing fashion
by Riegelman, Nancy.
* Fashion design illustration : women
by Ireland, Patrick John
*Fashion Sketchbook
Bina Abling,
(was one of our reccomended textbooks for school, but was too expensive to buy so I didnt

)
God bless