Iam trying to build a 20 x 20 basic shed or building with no windows just one door for my visually impaired gr?

20 x 20 grampa wants it ! He's blind and needs my help can u show me what to do so gramps doesn't think I dumb ..lol. Only 1 door he wants he ordered the wood for it just needs to go together..has rafters already made..he keeps saying 20 x 20 and nothing else ..please help blue print or pictures anything he keeps asking me to build but kinda hesitant to stary cutting up all the wood he bought the package from rona to build a 20 x 20 he says he wants it 16inch centres I don't know how to do the corners studs the rafters r built to fit a 20x 20 thanks. Asap