First, choose a darkish green and draw the outlines of the parts of the outfit that AREN'T going to be transparent, such as the topmost portion of the skirt and her bust.

Fill in those areas and shade them in whatever way you prefer. I finished the top of the skirt with a few drapey folds, but you can leave those out if you want.

Now, scrolling down in the Custom Colors screen (Colors > Edit Colors > Define Custom Colors), choose a medium to dark greyish or brownish-green. Grey hues on flesh make the transparency look more realistic and like there's skin underneath. Make the straight outline of the skirt (folds are more difficult to master in transparent shading), and color in the outlines of the doll's midriff and some armgloves.

If you are more inexperienced with Paint and custom colors, this palette that I used may help you:

Fill in the area behind the legs with a medium regular green. Since there's no flesh underneath or behind this area of the skirt, you want it to look like it has further dimension. Now take your brown-green and lighten it a little to fill in the outlines of the doll's legs, then lighten it a little more to fill in the darkest shading on her arms, midriff, and legs. Another lighten will cover the rest of the areas.

Start making highlights on her legs, arms, and midriff by making your brown-green a little lighter again. Continuing to highlight gives the transparent material a shiney, gauzy effect.

I've zoomed in to show the effect of more highlighting. Don't forget to lighten the back areas of the skirt, or it will look funny, as though the back of the skirt is solid and the front is transparent!

For detailing, I made straps on her top, a solid border on the bottom of her skirt, and golden sparkle patterns on the transparent part of the skirt. You don't have to add these, but they do tend to make things prettier ;)

As finishing touches on the outfit, I added sheeny golden borders here and there. Add the hair, shoes, and jewelry (or whatever else you can think of!) and you're done! :D