I am writing my resume and confuse in 'Writing' technical proposal sentence. What is the proper word of it for technical proposal? Is it writing, creating, developing, or other?
"Writing" works just fine. There isn't a "more formal" version of "writing" that I can think of. Honestly, I really have no idea why you would be worried about this. If you're putting text on a document, then you're writing, whether it be by pen or on a computer through a keyboard.
"Developing" implies that you made a finished product, so unless you made money off of your technical paper, it wouldn't be applicable -- and even then it would not be idiomatic. "Creating" is applicable, but not very idiomatic either, because it could mean that you did not actually finish the technical document.