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That is the hard part: learning how to use software that will let you design the thing you want to print. It tends to have a steep learning curve, and the types that don’t, in my experience often lack the more advanced features you will probably want to design model parts.
Of course, if you can download a file that somebody else made for the thing you want to print, this isn’t a problem — you would only need to learn how to send that to the printer.
