it can be hard trying to recreate clothes from a drawing like this. Because the cartoonist can make that fabric do anything he/she wants!

First, you can't make that cape and that top be one piece, or you won't be able to get it on. I'd make the cape separate, and then have it simply lie on top of a unitard with a center white stripe.

For the cape itself, I would consider using an invisible zipper that's installed in the center back going down from the neck just enough to make the opening wide enough for your head.

I'd use white ribbon or some other white material to sew on all the trim. And then at the center front, I'd use a snap to make sure the cape's center stripe lines up with the unitard's. (But remember that you can also position, then baste a few stitches to hold it there, if you don't like the bulkiness of the snap(s) .)

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This cape has a stand-up collar, which you could make taller. And you could cut away the front part of the cape to make that curve to the shoulder (start with it shorter, and then draw your line for that bottom hem once you've assembled it and can try it on)

This dress pattern could be adapted--just make the yoke part for the front, and draw the funnel neck higher. Then make the back part longer by joining it with your cape pattern.

Or see if you can find a turtleneck blouse pattern that is made for woven (not knit), and use the neck part. You can just make the top part of the bodice, and then fuse on your cape pattern for the back.