How can I improve my clothing?

So, I’ve been designing clothes with paint.net for a long time now, and I was wondering why my clothes are so bad still? People with months of experience design better, or at the same level as me. I also want to know the best teqniques of resizing images. I usually just press Ctrl + R to resize it, but it still comes out kinda pixelated. What are you best ways of shading? I have watched some youtube videos to improve on my work but I want to get advice and tips on it. Fair warning my designs are extremely bad.
I USE PAINT.NET BTW.

Some of my recent designs

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