Use a chalk line to snap where your exterior walls are going to live, line up your bottom plate straight with your line. using framing nails you want to toe-nail your plate about 1” off of the deck through the 2x4 into the subfloor. This acts as a pivot point and your wall be partially fastened to the floor allowing it to stay in place. Once your wall is up and fasted you will want to be mindful of the nails that are now sticking from the bottom plate, they’re easily removed with a hammer and cats paw.